• No results found

Small-scale biogas and greenhouse system

N/A
N/A
Protected

Academic year: 2021

Share "Small-scale biogas and greenhouse system"

Copied!
66
0
0

Loading.... (view fulltext now)

Full text

(1)

IN

DEGREE PROJECT TECHNOLOGY,

FIRST CYCLE, 15 CREDITS ,

STOCKHOLM SWEDEN 2017

Small-scale biogas and

greenhouse system

ROBERT ALEXANDERSSON

STEPHAN TRAN

(2)

TRITA -IM-KAND 2017:15

(3)

Small-scale biogas and greenhouse system

Robert Alexandersson

Stephan Tran

Supervisors:

Gunnar Bech

Anders Malmquist

(4)

Abstract

Greenhouse cultivation is a growing industry, especially in mild climates, much due to the ability to adjust the growing conditions and increased water utilization efficiency. The most important aspect on the cultivation is the indoor temperature. The variation in temperature is due to the Swedish climate where the highest and lowest outdoor temperature respectively varies greatly during the year. To enable optimal indoor climate additional heating is required during colder periods. Presently, most of the existing greenhouses utilizes combustion of fossil fuels for heating, which contributes to the climate change through the emissions of greenhouse gases. One way to circumvent this impact is to exchange the fossil fuels with biogas. Combining biogas production and greenhouse cultivation enables synergies and a more closed cycle of material flow can be achieved. However, this combination is rather immature due to lacking previous research, giving this report its main purpose, to examine the synergies and sustainability of combining a greenhouse with small-scale biogas production.

Initially, an extensive literature study was carried out followed by a simulation based on the obtained knowledge. The simulation was comprised of two greenhouses with different geometries, one with the shape of an arch with polyethylene-film cladding and the other with a sawtooth roof with glass cladding, both with two layers. The other properties such as internal area and volume are more or less the same for the simulated greenhouses. Useful data such as outdoor temperature, rainfall and solar irradiation etc. was obtained for the city of Enköping, Sweden. The calculations for the models were carried out in the program Microsoft Excel. In order to evaluate the feasibility of these models a reference greenhouse was studied, which had similar properties and conditions.

The optimal temperature for tomato cultivation is 20° C, and to maintain this level over the entire year it was found that the heat requirements were 89 500 kWh for the arched greenhouse and 94 400 kWh for the sawtooth greenhouse. In comparison with the reference greenhouse, the heat requirement was around 200 kWh per m2 and year less in the simulated greenhouses. Furthermore, it was found that around 31 800 kWh of cooling is required over the year (249 kWh per m2 and year) for the arched greenhouse and 30 900 kWh per year (241 kWh per m2 and year) for the sawtooth greenhouse, to keep the indoor temperature at 20 °C. Moreover, two to three possible harvests annually gives the yield of 3456-5184 kg tomatoes per year. Both the simulated greenhouses are feasible concepts, however the sawtooth greenhouse is a better option due to its increased longevity and lower contribution of greenhouse gas emissions over time. Furthermore, more research needs to obtain a fully closed cycle.

(5)

Foreword

This report is the result of a bachelor thesis comprising 15 ECTS credits, conducted at Royal Institute of Technology (KTH) in spring 2017, within the field of sustainable energy technology. The report constitutes a part of the Degree Programme in Energy and Environment at KTH. The content of the report was mainly based on an idea by Gunnar Bech, chairman for Innovationsverket in Gamleby. Acknowledgment and gratitude is addressed towards both Gunnar Bech and Anders Malmquist, Associate professor at the department for Energy Technology at KTH, for inspiration and advice during the work process.

Stockholm, May 2017

(6)

Table of contents

1 Introduction ... 1

1.1 Background ... 1

1.2 Purpose and goal ... 1

2 Literature study ... 2

2.1 Biogas ... 2

2.1.1 Biogas production on smaller scale ... 2

2.1.2 Utilization of small scale biogas ... 4

2.1.3 Current trend and potential of biogas in Sweden ... 5

2.2 Greenhouse ... 6

2.2.1 Shape and orientation ... 7

2.2.2 Greenhouse materials ... 8

2.2.3 Climate control in greenhouses ... 10

2.2.4 Sustainability aspect of the greenhouse ... 11

2.3 Horticulture/plant growth ... 13

2.3.1 Temperature requirement ... 13

2.3.2 Growth medium ... 14

2.3.3 Nutrient requirement ... 17

2.3.4 Water requirement ... 21

2.3.5 Humidity requirement ... 21

2.3.6 Carbon dioxide requirement ... 21

2.3.7 Light requirement ... 23

3 Method and model ... 25

3.1 Planning the cycle of the system ... 25

3.2 Dimensions of the greenhouse ... 26

3.3 System energy balance ... 28

3.3.1 Thermal mass ... 28

3.3.2 Biogas energy content ... 29

3.3.3 Energy balance ... 29

3.4 System material balance ... 32

3.4.1 Water ... 32

3.4.2 Biogas slurry and nutrients ... 32

3.4.3 Carbon dioxide ... 33

3.4.4 Crops ... 33

3.5 Location climate data ... 34

3.6 Sustainability aspect ... 34

3.7 Reference greenhouse for comparison ... 35

4 Result and discussion ... 36

4.1 Greenhouse conditions without climate control ... 36

(7)

4.2.1 Heating ... 37

4.2.2 Cooling ... 38

4.2.3 Electricity ... 39

4.2.4 Combustion engine power required for greenhouse operation ... 40

4.2.5 Biogas required for energy ... 40

4.2.6 Reference greenhouse system ... 41

4.3 Greenhouse material balance ... 42

(8)

Nomenclature 1 - Abbreviations

Abbreviation Expansion

BS Biogas slurry

CBS Concentrated biogas slurry

DRAPS Double recirculating aquaponics system GHG Greenhouse gas emission

HPS High pressure sodium

IPP Integrated production and protection IR Infrared radiation

LCA Life cycle analysis LED Light emitting diode MSW Municipal solid waste NFT Nutrient film technique

PAR Photosynthetically active radiation PC Polycarbonate

PE Polyethylene

PMMA Polymethyl methacrylate PV Photovoltaic

RAS Recirculating aquaculture system SEK Swedish krona

SMHI The Swedish meteorological and hydrologic institute SRAPS Single recirculating aquaponics system

(9)

Nomenclature 2 - Variables

Variable Representation Unit

A Area m2

cp Specific heat capacity J/(kg ·°C)

ɛ Emissivity coefficient -

E Energy J

h Heat convection coefficient W/(m2·°C) k Heat conduction coefficient W/(m ·°C)

L Characteristic length m

Λ Transmission and ventilation losses W/°C

m Mass kg

mg Gas mass g

M Molar mass g/mol

µ Dynamic viscosity kg/(m · s)

n Moles mol

Nu Nusselt number -

P Pressure Pa

Pr Prandtl number -

Q̇in Influx of heat power W

Q̇out Outflux of heat power W

Q̇rad Radiation heat power W

R Gas constant: 8.314 (Pa · m3)/(mol · K)

Re Reynolds number -

ρ Density kg/m3

σ Stefan-Boltzmann constant: 5.67 · 10-8 W/(m2 · K4)

T Temperature °C

Tamb Ambient temperature °C

Tg Gas temperature K

Tinf Reference temperature °C

Tsurf Surface temperature K

Tsurr Surronding temperature K

τ Timestep h

τB Time constant of structure s

u Velocity m/s

U Overall heat transfer coefficient W/( m2·°C)

V Volume m3

V̇ Ventilation rate m3/s

(10)

1 Introduction

1.1 Background

Greenhouse cultivation is a growing industry, especially in mild climates, much due to the ability to manipulate growing conditions and increased water utilization efficiency (von Zabeltitz 2011, 1). There are many aspects to consider in greenhouse cultivation, and one of the foremost important aspect is additional heating during cold seasons, to enable an optimal indoor climate. Today a large amount of heated commercial greenhouses utilizes combustion of fossil fuels for heating, which adds to the environmental impact through greenhouse gas emissions contributing to climate change. One way to circumvent this impact is to exchange the fossil fuels with biogas. A further benefit of having biogas generation and greenhouse cultivation in conjunction is that resources from biogas generation and combustion have the potential to enhance crop growth, through the use of biogas slurry and carbon dioxide. This way a more closed cycle of materials flow could be achieved.

Presently, the technology for biogas production conducted at larger scales, commercial scale are more mature compared to small-scale biogas systems. In Sweden on average one new small-scale biogas facility is constructed annually. However, the technology is not unfamiliar since in Germany on average 1500 new small-scale biogas facilities are constructed annually. The difference is the existing policy, mostly the economical aspect, where the farmers in Germany are guaranteed a fix price for each produced kWh that sells to the local grid. This does not apply to the farmers in Sweden, additionally, the price for biogas has been lower in Sweden compared to Germany (Edström & Nordberg, 2004). With low price for biogas there is no direct incentives to sell the produced energy, thus giving the conclusion that it instead should be reused in the local area and its facilities.

1.2 Purpose and goal

This report was created in joint collaboration with Gunnar Bech from Innovationsverket and the faculty of sustainable energy technology in KTH, in order to explore utilizations of biogas in a rural area. The main purpose of this report is to examine the synergies and sustainability of combining a greenhouse with a biogas facility in a rural area. In addition, the lack of previous research of this kind of system makes this interesting to examine. To fulfil the purpose of this report the following goals were set up:

● To find synergies between small-scale biogas production and greenhouse operation

● To strive towards a closed loop system (recirculation) by utilizing the ascertained synergies ● To identify the contribution of greenhouse gas emissions of greenhouse materials and means

of lowering the them

(11)

2 Literature study

2.1 Biogas

Biogas is a renewable and high methane content combustible gas. It can be produced through the decomposition of organic material by microorganisms under anaerobic conditions, in a process called anaerobic digestion. This production method can be utilized to obtain biogas from landfills, wastewater treatment and other organic materials, such as manure and crop residues from agricultural activity (Energigas Sverige, 2014). Biogas mainly contains methane (CH4) and carbon dioxide (CO2), with the

following constituents in considerably smaller concentrations: nitrogen (N2), hydrogen (H2), hydrogen

sulfide (H2S) and oxygen (O2). Furthermore, raw biogas is usually saturated with water vapor (Avfall

Sverige, 2015). The volume percentages of the compounds in biogas are presented in table 2.1.

Table 2.1 Estimated volume percentages of various molecular compounds in newly produced biogas from organic waste (Christensson et al. 2009, 13; Jørgensen 2009, 4):

Molecular compound Volume % in biogas

Methane (CH4) 55-80

Carbon dioxide (CO2) 20-45

Nitrogen (N2) 0-1

Oxygen (O2) Trace

Hydrogen (H2) Trace

Hydrogen sulfide (H2S) 0-2000 [ppm]

The CH4 is the compound that contributes to the energy value of the biogas, where 1 normal cubic meter

(Nm3) of CH

4 has an energy value of 9.97 kWh. The unit of Nm3 is given by the volume of a gas at 0

℃ and atmospheric pressure. Given that the amount of CH4 may vary, the energy density of raw biogas

varies between 4.5 and 8.5 kWh/Nm3 (Ibid). CO

2, H2S and water content are corrosive factors and the

amount thus determines the corrosiveness of the biogas. For distribution of gas in grids it is important to lower the water vapor content in the biogas to reduce the risk of grid blockage - the blocking of biogas distribution pipes. This can be achieved by condensing it through a pressure increase or temperature decrease and may be desirable for the use of biogas in apparatuses, since condensed water may cause corrosion damage to them (Christensson et al. 2009, 14). The chemical reaction for biogas complete combustion of methane is presented in reaction 2.1.

(2.1) CH4 + 2O2 → 2H2O + CO2

2.1.1 Biogas production on smaller scale

(12)

undergoes pre-treatment, where undesired or harmful components are separated and the material is ground to increase the digestion rate. The treated digestate then enters the digester, the chamber where anaerobic decomposition takes place and biogas is generated. The digester is commonly heated to temperatures of around 37 ℃ (for mesophilic conditions) or 55 ℃ (for thermophilic conditions) and has a continuously running stirring device in order to blend the digestate and ensure that decomposition and heat distribution is uniform. After the main digestion process the biogas slurry may be placed in a post digester if the digestion is not fully complete. Both the main and post digestion chambers produce biogas which is lead through pipes to end usage or refinement instances (Christensson et al. 2009, 21-33).

Figure 2.1. A schematic diagram of small-scale farmyard biogas production system.

Biogas can be stored in a gasometer, but the use of it is limited in small-scale applications. Instead the digester may hold the same function through the use of a flexible membrane for biogas collection inside the chamber, however, this method yields a storage capability of a considerably shorter term. Regardless of storage capability a flaring stack is connected to the digester output tubes so that the biogas may be flared when its production exceeds storage and use. When biogas is flared the CH4 is converted to CO2

and H2O and thus the global warming potential can be reduced as opposed to release of biogas into the

(13)

All organic material may be digested with varying time requirements. Table 2.2 illustrates biogas yield and methane content of three different organic materials: cellulose, protein and fat. As can be seen, fat yields roughly double the amount of biogas per unit weight and also attains the highest energy value, defined by the methane content (Jørgensen 2009, 19).

Table 2.2 Biogas yield at STP1 and methane content, upon complete digestion of three various compounds: cellulose (carbohydrate), protein and fat. The general chemical processes are also shown (Jørgensen 2009, 19).

Organic material Process ml biogas/g ml CH4/g CH4 (%) Cellulose C6H10O5 + H2O → 3CH4 + 3CO2 830 415 50.0 Protein 2C5H7NO2 + 8H2O → 5CH4 + 3CO2 + 2(NH4)(HCO3) 793 504 63.6 Fat2 C57H104O6 + 28H2O → 40CH4 + 17CO2 1 444 1 014 70.2

1 STP = Standard Temperature and Pressure (0 ℃ and 1 atm) 2 Fat in the form of glycerol trioleic acid

2.1.2 Utilization of small scale biogas

Among small-scale biogas facilities in Sweden the most common utilization of the biogas is heating of directly nearby farmyard buildings with the use of a combustion or gas engine. However, some facilities also use the energy from combustion of biogas to generate electricity, with 30-35 % efficiency, and it may then be used within the farmyard or alternatively sold and distributed out through the grid (Naturvårdsverket 2012, 77; Energimyndigheten 2016, 9).

In Sweden micro producers of renewable electricity, i.e. individuals such as homeowners with small power generation abilities, may sell electricity to electricity trading companies or companies that own the grid. The possible revenue varies from company to company, but it is often 0.1-0.5 SEK/kWh (Energimyndigheten, 2017a). From January 1st 2015 the micro producer can apply for a tax reduction

of 0.6 SEK/kWh, but no more than 18 000 SEK, corresponding to 30 000 kWh produced annually. Also, electricity sold must not exceed electricity purchased. If revenues from electricity sales are beneath 30 000 SEK/year the micro producer may also be exempted from value-added tax (VAT) (Skatteverket, 2017). The two of the largest energy companies in Sweden, Fortum and Vattenfall, currently both pay the spot price of electricity at the Nordic electricity market Nord Pool for micro produced electricity (Fortum, 2016; Vattenfall, 2017). Furthermore, producers of renewable electricity may be granted one green certificate from the state for every MWh of electricity produced. The certificates can then be sold on the open market to other electricity providers (Energimyndigheten, 2017a). The average price per certificate between March of 2016 and March of 2017 was 146.89 SEK (Energimyndigheten, 2017b).

(14)

systems, with few exceptions, due to economic reasons. It is instead an application more commonly in conjunction with larger operations. One method for which upgrading of biogas from small-scale operators could be realized is the implementation of local gas grids. With gas grids small biogas producers may have their gas distributed to centralized upgrading facilities (Christensson et al. 2009, 40).

In 2013 an exploratory project was initiated to investigate the market conditions for expanding an existing local biogas grid in the Mälardalen region in Sweden. It was found that the production of biogas in the region (337 GWh) must increase as well as the biogas demand. Demand was found to increase in the transportation sector by 1 TWh to 2020 and 1 TWh demand was further concluded possible within local industry. Meanwhile the biogas production potential in the region was estimated to 4 TWh (Forsberg, 2014).

The exhaust from boilers or combustion engines for heat and electricity generation could be utilized for CO2 enrichment in greenhouses. This could aid in boosting crop growth and reduce or eliminate the

need for bought CO2 from another source (Christensson et al. 2009, 40). Another residue from the

biogas system, biogas slurry, is rich in compounds with high nutritional value for plants and could thus be used for fertilization. Dependence on bought fertilizers can then be reduced and a more closed loop of resources may be achieved. In 2015 all of the biogas slurry produced in farmyard facilities was used as bio fertilizer (Energimyndigheten 2016, 19). In a recent study it was concluded that biogas production on organic farmyards come with the potential for farms to gain positive energy balance, reduce GHG emissions and offer self-sufficiency regarding organic fertilizer nitrogen (Pugesgaard et al., 2014).

2.1.3 Current trend and potential of biogas in Sweden

In Sweden 2015 a total of 1 947 GWh biogas was produced. The different ways of producing biogas is presented in table 2.3. The production of biogas in Sweden has had a continuous growth since 2006 to 2015, as shown in table 2.4. Compared to the year 2014 the biogas production for small scale farm increased with 14 % (Energimyndigheten 2016, 12).

Table 2.3 Different ways biogas was produced in 2015 (Energimyndigheten 2016, 7).

Different applications of biogas Percentage of total production of biogas [%] Digestion facilities 44

Waste water treatment 36

Landfills 9

Industrial facilities 6 Small scale farm production 3

(15)

Table 2.4 Different ways biogas was produced in 2015 (Energimyndigheten 2016, 7).

Year 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

GWh 1 213 1 258 1 359 1 363 1 387 1 473 1 589 1 686 1 784 1 947 During 2015 biogas produced from small scale farms only consisted of 50 GWh, and of this 34 % was converted to heat, 16 % was converted to electricity and the remaining gas went to other applications, e.g. upgrading of biogas (Energimyndigheten 2016, 15). Furthermore, according to the report “EU Reference Scenario 2016 - Energy, transport and GHG emissions trends to 2050” by the European Commission through an analysis of future trends, shows that the quantity of GWh in terms of electricity will increase until 2050, as shown in table 2.4 (European Commission, 194).

Table 2.5 Different ways biogas was produced in 2015 (Energimyndigheten 2016, 7).

Year 2015 2020 2025 2030 2035 2040 2045 2050 Electricity generated by source (GWh) 14 846 17 307 16 195 20 890 23 299 22 662 26 440 27 121 Fuel inputs to thermal power generation (GWh) 4 886 4 556 4 703 5 437 5 713 5 689 6 209 6 392

2.2 Greenhouse

A greenhouse is a facility for cultivating plants and crops where the growth environment is protected from the outside environment in addition to where growth and climatic conditions may be manipulated. This means that greenhouse cultivation can be made largely independent of outdoor environmental conditions (Reddy 2016, 13). The greenhouse is constructed with transparent materials for its walls and roof. This allows for high transmittance of sunlight while simultaneously trapping heat inside of it, with its structure having low transmittance for infrared radiation (IR), which lowers heat transfer from crop environment to the outside environment. As such greenhouses may be used to cultivate all year round in regions where the growing period is limited to few months of the year. For high yield and quality of greenhouse crop production the following is required (von Zabeltitz 2011, 1-2):

● Appropriate greenhouse structure

● Good mounting, installation and maintenance of the system ● Knowledge and skill of workers

● Efficient management of production

● Efficient climate control during summer/winter

(16)

2.2.1 Shape and orientation

Greenhouses can be made in several different shapes and both their shape and orientation have an impact on the amount of solar irradiation reaching the greenhouse crops. The sunlight that is transmitted through the greenhouse walls varies with the time of day as well as the season. Table 2.6 presents the mean transmittance for five different greenhouse types in north-south and east-west orientation: (a) saddle roof with 15° inclination, (b) saddle roof with 25° inclination, (c) saddle roof with 35° inclination, (d) saw tooth or shed roof with 25° and 65° inclination and (e) round arch with vertical side walls. As can be seen from it light transmittance increases with the inclination of saddle-type roofs and the saw tooth and curved roof types have the highest transmittance, although the saw tooth type has a larger surface and is thus subject of greater heat loss. A north-south orientation yields high transmittance in summer, whereas an east-west orientation offers high transmittance during winter (von Zabeltitz 2011, 138).

Table 2.6 Mean transmittance (in %) of different greenhouse types with North-South and East-West orientation in the months of December and June (von Zabeltitz 2011, 138).

Greenhouse type December June

North-South East-West North-South East-West

(a) 36 % 37 % 58 % 56 % (b) 43 % 47 % 62 % 57 % (c) 52 % 62 % 67 % 61 % (d) 52 % 70 % 69 % 78 % (e) 52 % 65 % 72 % 70 %

For greenhouse cultivation in northern mild climates it is necessary to weigh the structure type, insulating properties and optical characteristics of the greenhouse for optimal growth temperatures and economy. It is desired to minimize the heat requirement during cold months as well as to minimize cooling requirements during warm months (Maslak 2015, 26, 28).

(17)

climatic conditions of the indoor environment. However, that brings trade-offs in the form of larger surface area resulting in increased heat transfer and higher greenhouse build costs (von Zabeltitz 2011, 60). For numerous varieties of tomato crops, most fruits develop in the first 2 meters of the crop (Schwarz, 1995) and most new greenhouses for tomato cultivation have heights of 3.5 - 6 metres to accommodate for wiring support of plants and artificial lighting above them (Peet & Welles 2005, 259-260).

2.2.2 Greenhouse materials

The greenhouse cladding is the part of the structure consisting of walls and roofs and that transmits sunlight into the area enclosed by the greenhouse. The character of the cladding material determines the transmittance as well as other aspects of the material, such as adequate strength and thermal resistance. Cladding materials should have a high transmittance of PAR (photosynthetically active radiation), low transmittance of IR (infrared radiation), low ageing of UV (ultraviolet) radiation, low accumulation of dust and dirt, no dropwise condensation and have high endurance against the wind. The avoidance of dropwise condensation is desired due to the increased danger of diseases and lowered light transmission. This can be realized by using cladding with film condensation properties. For arched roof greenhouses one solution is also to create an upward pointing edge on the top the arch structure, which is commonly known as a venlo type roof structure. This will make water drops run down along the cladding material, instead of potentially dripping down from the mid-section of the roof structure (von Zabeltitz 2011, 145, 162). The light transmittance is one of the most limiting factors for yield maximization in almost all regions. In a study in North-America in 1998 it was found that a 3 % decrease of transmittance resulted in 1 % lower tomato yield, whereas a study in the Netherlands 1984 reported 1 % lower yield when transmittance decreased by 1 % (Peet & Welles 2005, 290-291).

There are three main types of cladding materials used for greenhouse operations: glass, rigid plastic and plastic film. Of these, very common rigid plastics are polymethyl methacrylate (PMMA) and polycarbonate (PC) sheets and the most common plastic film consists of polyethylene (PE). These all come with different advantages and disadvantages, respectively (Maslak 2015, 27; von Zabeltitz 2011, 147):

● Glass:

Glass has the highest light transmittance of all above mentioned greenhouse cladding materials, usually more than 90 % for single panes and above 80 % for double panes. The material does not deteriorate over time. It is however rather heavy, so expensive and adequately supportive structures and frames are necessary. It is also very brittle, making it susceptible to impacts. Greenhouses can have double glass panes for increased thermal resistance. The heat conduction coefficient for glass is 1 W/(m*K) (The Engineering Toolbox, n.d.).

● Acrylic plastic:

(18)

● PC-sheets:

Polycarbonate double sheets may have up to 83 % light transmittance. They are flexible and lightweight which make them attain desired shapes easily and lowers the need for strong supportive structures. The material is vulnerable to degradation from ultraviolet radiation, which makes it turn yellowish over time, but this process can be slowed down with special treatment to suppress deterioration from UV-radiation. PC-sheets have a thermal conductivity of 0.19 W/(m*K) (The Engineering Toolbox, n.d.).

● PE-film:

Polyethylene film may have almost as high transmittance of light as glass, around 90 % for single layers and around 80 % for double layers. It is the least expensive of the mentioned materials, come in wide widths and is flexible and lightweight, making it easy to install. Like PC it degrades over time due to exposure of UV-light, which may yield a short lifetime of only 2 years. PE can however be UV-treated as well and can then have lifetimes of up to 10 years (SolaWrap, 2016). The material has a thermal conductivity of 0.33 W/(m*K) (The Engineering Toolbox, n.d.).

The cost of greenhouse components and materials vary depending on level of technology. With highly technological properties the controlling capability is extensive and crop cultivation can be made very efficient, but then the initial cost of components is usually high. Measures can be taken to lower initial cost through downgrading of technological level, however control and further growth efficiency may be negatively impacted (table 2.7) (Ponce et al. 2014, 64).

Table 2.7 Materials and components for low to high tech greenhouses that last for at least 10 years and their approximate costs (Ponce et al. 2014, 65).

Technology level of greenhouse

Low Medium High

Structure Wood or steel Steel Steel or aluminium

Cladding Single layer PE film Double layer PE film or rigid plastic

Glass, PC or PE

Heating No Yes, air heating Yes, hot water pipes

Cooling Yes, passive Yes, passive and/or active Yes, evaporative cooling

Growth medium Soil Soil or soil-less substrate Soil-less substrate

Irrigation Manually controlled drip irrigation

Partly automatically controlled drip irrigation or hydroponics

Fully automatically controlled drip irrigation or hydroponics

(19)

2.2.3 Climate control in greenhouses

The greenhouse climate is a complex and nonlinear system, and is the main factor for optimal growth conditions for the plants. Factors with great significance of the internal climate of a greenhouse is light, temperature, humidity, CO2, water, fertilizer, etc. However, light and temperature are of significant

importance for the cultivation system climate (Mahdavian, M et al. 2017, 835). The greenhouse is a complex thermodynamic system where the internal temperature may become high due to trapped incident solar radiation (Maher, A et al. 2016, 1243). The internal temperature and humidity respectively vary greatly during the day, thus regulating systems are essential to obtain optimal cultivation conditions within the greenhouse. External disturbances such as weather conditions affect the indoor climate of the greenhouse greatly, which however can be easily adjusted (Ibid, 1247). There are different methods of regulating the heating and cooling demands in a greenhouse to obtain desired indoor temperature, both active and passive technologies. Using conventional photovoltaic (PV) modules are convenient because of their multifunctionality, e.g. the converted electricity from solar radiation can be stored in batteries and subsequently used in different appliances to regulate the indoor climate of the greenhouse, such as lamps, coolers, heaters etc. In addition, it is possible to enable both thermal and energy generation in photovoltaic/thermal modules, thus also increase the efficiency. Water is a common media that is used as energy carrier in these systems, but air can also be used. In temperate climates the air based system is more cost effective, but also enables a more simple system than water based (Cuce et al. 2016, 38-39).

Furthermore, thermal energy storage is also a common method used to stabilize the indoor temperature. The main factor of this technology is the storage material and the used container. The most common material used to store heat is gaseous materials such as air, liquids such as water and solid material such as phase change material (PCM). In addition, soil is also considered in thermal energy storage as it has similar characteristic properties as solid materials. Soil is usually used as a heat sink to reduce internal temperatures in greenhouses. In climate with higher demand for cooling, soil is a good option as a passive cooling application (Ibid, 43, 45).

Moreover, another technology with many utilities is heat pumps which can be used both for heating and cooling the greenhouse whenever required. Heat pumps can also be used to control the relative humidity of air in the greenhouse. The heat pumps are usually preferably implemented with a ground source with the heat exchanger pipes horizontally in greenhouses (Ibid, 46-48).

A common passive cooling technology that is used in the greenhouses to regulate the temperature is through wind catchers. Which has the function to lower the temperature inside by flowing fresh air through natural convection from the roof of the building. The pressure difference is an affect due to difference in temperature with outdoor and indoor, resulting in warm air inside is pushed upward and out of the building replaced with cooler air thus the temperature drops. However, since the cooling technology using wind catchers is passive and is both open and in direct contact with the greenhouse, there is an increased risk of admittance of pests to the interior of the greenhouse (Ibid, 51).

(20)

However, the CO2 enrichment affects the air temperature in the greenhouse since ventilation decreases

the temperature but simultaneously also decreases the CO2 concentration in the air. In warmer climates

this can be a problem, and a more precise balance between ventilation and desired CO2 level is required.

There are various ways to obtain optimal climate considering the ventilation and the CO2 level in the

greenhouse (Amir, R et al. 2005, 619-620).

A common way is through forced ventilation in combination with injection of CO2. In a study a few

tests were made with different modes of ventilation and all with the same rate of injected CO2. The

modes consisted of an injection rate of CO2 of about 1.8kg/h to a concentration of 1000 ppm and four

different ventilation modes: high ventilation, low ventilation, alternating high and low ventilation and lastly, adjusting the ventilation to the heat load. The last ventilation mode gave the best results, with high levels of CO2 concentration and low electricity requirements considering the usage of ventilation

(Ibid, 621-623).

2.2.4 Sustainability aspect of the greenhouse

Some aspects of greenhouse cultivation have more significance for the environmental footprint than others. The choice of building materials has large significance for the contribution of greenhouse gas emissions (GHG). The building materials consist of greenhouse anchoring or foundation, greenhouse frame or structure material and cladding material, and their contribution to GHG emissions are presented in table 2.8. Furthermore, the impact of continuous greenhouse operation and maintenance must be assessed. These aspects consist of utilization of heat, electricity, water, pesticide and transport services (von Zabeltitz 2011, 118-120).

Table 2.8 Greenhouse gas emissions in kilograms carbon dioxide equivalents (CO2e) per kilogram of various

greenhouse structure material (Ruuska 2013, 24; Al-Amin et al. 2011, 367; Boustead 2005a, 9; Boustead 2005b, 9). Building material CO2e [kg/kg] Concrete 0.442 Aluminium 9.964 Steel 3.778 Glass 1.230 Polyethylene 2.130 Acrylic plastic 5.900 Polycarbonate 6.000

(21)

showed that the glass houses had up to 64 % higher heating requirements than the plastic film ones (von Zabeltitz 2011, 129, 131, 135).

In a study from 2013 the environmental impact of tomato cultivation in a glass greenhouse in Central Europe was studied using the LCA-methodology, in order to create a user friendly environmental impact calculator for greenhouse production systems. Parameters included were greenhouse structure, auxiliary equipment (for example growth medium), climate control system, fertilizers, pesticides, waste management and transport. Environmental impact categories consisted of air acidification, eutrophication, global warming, photochemical oxidation (e.g. ozone depletion) and water use. The greenhouse having an expected life time of 15 years and being heated by natural gas it was concluded that the operation of the climate control system was the main burden in all impact categories and overwhelmingly so in global warming potential (table 2.9). The next significant impacts were from structure and fertilizers (Torrellas et al. 2013, 188, 190-192).

Table 2.9 Impact from different greenhouse parameters on four impact categories per tonne tomato produced in a glass greenhouse in Central European climate conditions.

Impact categories 1 Air acidification [kg SO2 eq] Eutrophication [kg PO4-3 eq] Global warming [kg CO2 eq] Photochemical oxidation [kg C2H4 eq] Structure 0.282 0.087 47.19 0.013 Climate control 2.624 0.0698 1 823 0.196 Aux. equipment 0.063 0.014 8.482 0.003 Fertilizers 0.280 0.054 47.64 0.002 Pesticides 0.001 0.001 0.238 0.000 Waste 0.003 0.001 0.706 0.000 Total 3.252 0.8550 1 928 0.2150

1 Water use values were not given separately for each greenhouse parameter, but instead the total value of 14.06

m3 per tonne tomato yield was stated.

The energy for the greenhouse system can be supplied by local cogeneration of heat and electricity from biogas combustion. Biogas releases GHG emissions in the form of CO2 upon combustion, but the carbon

comes from plant matter that previously fixed it from atmospheric CO2during growth. Biogas can thus

(22)

Electricity may be alternatively or supplementary supplied to the above process by the electric grid. GHG emissions per kWh may then vary depending on the source of generation. Sweden is in the Nordic electricity market and emissions can thus be derived from its electricity mix, which currently is 100 g CO2e per kWh on average (Svensk Energi, 2016).

2.3 Horticulture/plant growth

2.3.1 Temperature requirement

Crops such as tomatoes are warm-season species and require greenhouses for year round cultivation in mild climates, due to the commonly cold temperatures over the year. Specific temperature requirements are illustrated in table 2.10 (von Zabeltitz 2011, 29-30). In a study on how temperature affected tomato growth, tomatoes were grown in constant temperatures of 14 ℃, 18 ℃, 22 ℃ and 26 ℃. It was found that the crops grown at 18 ℃ and 22 ℃ produced normal fruits, whereas at 14 ℃ growth was severely reduced, resulting in tomatoes of no marketable value. Crops grown at 26 ℃ had a poor appearance and suffered from abnormal vegetative trusses (Adams et al. 2001, 871).

Table 2.10 Recommendations on greenhouse temperature settings for plant growth (von Zabeltitz 2011, 29-30).

Condition Temperature of occurrence []

Comment

Frost 0 Plants may succumb to frost. Risk of temperatures lower than 0 ℃ can be neglected when minimum daily outside temperature is 7 ℃.

Cold temperatures <12 Growth, quality and yield of vegetables in greenhouses are negatively affected. For a tomato crop the temperature should not be lower than 15 ℃.

Optimal daytime temperature 22-28 Optimal night time temperature 15-20

Overall optimal temperature 17-27 When only relying on heating of the greenhouse from solar radiation, suitable daily outside average temperatures are 12-22 ℃.

The optimal temperature range for tomato growth can be set to 18-22 ℃.1

Hot temperatures >30 Growth, quality and yield of vegetables in greenhouses are negatively affected. Mean maximum temperature 35-40 The maximum temperature for a tomato

crop is 35 ℃.

(23)

For cultivation of tomatoes the optimal greenhouse temperature will fall within the range 18-22 ℃, accordingly. Since a daily average temperature of 12-22℃ may indicate greenhouse temperatures of 17-27 ℃, daily average temperatures of at least 12 ℃ will be required for optimal tomato growth without using biogas for additional heating. According to Swedish Meteorological and Hydrological Institute (SMHI) the average temperatures in Mälardalen, Sweden, in June, July and August of year 2016 were 15 ℃, 17 ℃ and 17 ℃, respectively, while the average temperature over the year was 7-8 ℃ (SMHI, 2017). The necessity of additional heating will thus be unlikely during the summer period and instead needs to be increased during winter and parts of spring and fall. During warm and sunny summer days the greenhouse temperature may exceed optimal cultivation temperature and thus some method of cooling is required to be adopted.

2.3.2 Growth medium

There are various methods to grow plants, either in soil or without soil. Soilless horticulture is also known as hydroponics, where roots are submerged in a solution enriched with nutrients. All the necessary minerals are dissolved in the solution (Orsini et al. 2013, 714). The solution can be used with or without an external artificial medium, e.g. sand, gravel, mineral wool, peat moss or sawdust etc. with the purpose of providing a more solid support. Hydroponics are suitable for indoor horticulture and the system of hydroponics uses as little as 1/20 of the amount of water compared to regular outdoor horticulture based on soil, mostly due to evapotranspiration. In hydroponic horticulture the soil borne pest and other pathogens can be neglected. Thus, an increase in crop yield and quality can be expected (Lakkireddy et al. 2012, 29, 31). A common method in hydroponics is nutrient film technique (NFT), and is when the roots of the plant are suspended in a channel where a solution of nutrients is able to flow pass. The channel is usually inclined so the nutrient solution can flow through the whole system without external forces. This method requires less nutrient solution compared to other and in addition, also has a relative low cost when constructing (Gunning et al., 2016). An example of trough dimensions for tomato cultivation is shown in figure 2.2.

Figure 2.2. A two-dimensional bird view schematic of a trough used for soilless cultivation of tomato crops. All values are in centimeters (cm) (Rutledge 1998, 9).

(24)

oxygen and thereby increase both the metabolic process and the growth-rate up to 10 times. In aeroponics the growing occurs without a growing medium unlike hydroponics which uses water (Lakkireddy et al. 2012, 30).

The yield for tomatoes in aeroponics is, as in hydroponics, much higher than for tomatoes grown in soil. The root aeration in aeroponics are a major factor leading to a higher yield compared to a system of hydroponics (Ritter et al. 2000, 132). Moreover, in hydroponic culture the root aeration is rather poor and is only able to solve 8.7 mg O2 in one liter of water. For tomato horticulture intermittent injections

of nutrient solution has an advantageous effect by decreasing the temperature around the root, especially for horticulture in greenhouses during summer. During the day the frequency of injecting nutrient solution is higher compared to the night, due to the absorption trends. The excess of the unused nutrient solution can be reused after a process of filtration and disinfection with UV irradiation. The unused nutrient mixes with the rest of the nutrient solution and can then be reused. With the use of aeroponics a saving of around 18% of nutrient solution can be made compared to hydroponic culture in mineral wool with recirculating system (Komosa et al. 2014, 164, 166, 174). The same amount of nutrients was used in both the aeroponic and the hydroponic (mineral wool culture) system and is presented in table 2.11 (Ibid, 165).

Table 2.11 Nutrients used in both aeroponics and hydroponic system.

Nutrient Aeroponic/hydroponic [mg/l] N-NH4 <14 N-NO3 210.0 P 70.0 K 351.0 Ca 170.0 Mg 84.0 S-SO4 132.0 Na 22.7 Cl 42.2 Fe 1.68 Mn 0.54

(25)

of fresh water but significantly less than a traditional system (both for fish production and tomato production). There are two concepts of RAS, single recirculating aquaponic system (SRAPS) and double recirculating aquaponic system (DRAPS). SRAPS is a basic concept where the waste water from the fish tank goes directly to the plants without any external adjustments or refinement. The root zone of the plant with its bacteria cleans the waste water and subsequently flows back to fish tank to be reused. In this system the main input of nutrients is fish feed, and therefore, also indirectly the nutrients for the plants as well. Which gives a deficiency of nutrients for the hydroponic system and impedes growth (Suhl et al. 2016, 335-336).

The more complex system, DRAPS, which combines three biological systems, fish, plants and nitrifying bacteria. The three systems require different conditions, e.g. pH-value. The optimum range for both fish and nitrifying bacteria is between 7 and 9, while for the hydroponic system the pH varies between 5.5 and 6.5. There is a correlation between increased pH and decreased nutrients such as phosphorus, zinc, iron and manganese. In DRAPS the fish production and plant production are kept separated but connected through a 3-chamber-pit. With this combination optimal conditions regarding nutrients and pH-value can be applied for both systems. The system contains of a mechanical filter where the waste water from the fish tank flows, and subsequently goes through a bio filter with the purpose of nitrification by converting ammonium into nitrate. The water containing nitrate is recirculated back to the fish tank. Intermittently the effluent from the mechanical filter goes into the 3-chamber-pit and kept there until its use for the hydroponic system. However, before implementing the nutrition to the plant's optimal concentrations of mineral nutrients are added to obtain optimal plant growth (Suhl et al. 2016, 336-337). The required nutrients is presented in table 2.12 (Suhl, J. et al, 2016, 338).

(26)

The process between the mechanical filter, bio filter and the 3-chamber-pit is where the optimization can be made, to either favour the fish production, or the plant production. With one cubic meter of fresh water 1.55 kg fish (Tilapia) and 46.1 kg tomato fruit can be produced. Furthermore, with one kilogram of fertilizer about 10 kg more of tomato can be produced in aquaponics compared to a hydroponic system. The utilization efficiency of fertilizer is improved with around 23.6 % in aquaponics compared to hydroponics (Suhl et al. 2016, 336, 340-341).

Figure 2.3. Scheme of a double recirculating aquaponics system (Suhl et al. 2016, 337).

2.3.3 Nutrient requirement

Every plant needs a combination of nutrients to thrive, and for tomatoes there are at least twelve essential nutrients for a normal growth and reproduction. The nutrients can be separated into two groups, macro- and micronutrients, where macronutrients are needed in larger quantities compared to micronutrients where the requirement is far less. Excessive usage of nutrients that exceeds the requirements of the plant can reduce the tomato yield, degrade the surrounding environment and decrease the fertilizer-use inefficiency. The macronutrients consist of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S), and the micronutrients of boron (B), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and molybdenum (Mo). The tomato production can be either soil based or based in water, so called hydroponics. The soil usually consists of some micronutrients and some calcium and magnesium, while hydroponic system needs to be provided with all the essential nutrients (Sainju et al. 2003, 1).

(27)

Phosphorus, P: Phosphorus is an important nutrient that helps to initiate root growth of tomato and therefore, need to be in high concentration in the early stage of the process. The nutrient is also a constituent of nucleic acid. With improved growth stimulated by phosphorus the utilization of water and other nutrients in the soil are improved, and also promotes substantial growth of stem and healthy foliage. There is usually an abundant amount of phosphorus in the soil, because the nutrient absorbs easily in the soil and hardly leaches from the soil. The tomato takes up small amounts of phosphorus compared to nitrogen and potassium (Ibid, 2).

Potassium, K: Potassium is a nutrient that the plant needs in large amount and its main function is to activate enzymes and regulate pH of the tomato fruit. The nutrient also has a key part in regulating the quantity of the production of tomatoes per plant, since the nutrient stimulates early flowering and setting of fruit. Excessive amount of potassium reduces the availability of magnesium in the soil and as nitrogen, potassium is also soluble in water and can therefore leak into the groundwater (Ibid, 3-4).

Calcium, Ca: Calcium is as well amongst the nutrients that the tomato plant needs in large amount. Most of the soil contains sufficient amount of calcium for tomato growth, but deficiency occurs when the soil pH is below 4.5. The optimal range of pH in the soil is from 5.5 to 7.0. With excess of calcium the effects can be deficiency in micronutrients, such as iron and manganese (Ibid, 4).

Magnesium, Mg: The main function of magnesium is that it is a constituent of chlorophyll. Moreover, the mineral also contributes in fruit production. Deficiency of magnesium is common for greenhouse-grown tomatoes. However, the deficiency might not affect the fruit production unless the issue is severe (Ibid, 4).

Sulphur, S: Sulphur is an element of protein and amino acid. The mineral is usually applied in combination with N, P and K fertilizers, and therefore deficiency of this nutrient is rare. Through the atmosphere the tomato can absorb sulphur as sulphur dioxide (SO2) (Ibid, 4).

Boron, B: Boron can influence on the production of tomato flowers and fruits, by having an important role in the insemination and reproductive growth. Deficiency of boron can cause reduction of root growth and the deficiency triggers by increased value of pH in the soil and dryness around the root zone (Ibid, 4).

Iron, Fe: Iron is an element in many enzymes of the tomato. Deficiency of iron often occurs in soils with high pH and soilless medium. The solubility of iron in the soil decreases as the amount of excess of phosphorus increases (Ibid, 4).

Manganese, Mn: Manganese main function is to activate enzymes. The deficiency is as iron induced by high soil pH (Ibid, 5).

Copper, Cu: Copper is a constituent of oxidizing enzymes. Deficiency is more common in greenhouse-grown tomatoes or in soilless medium than field greenhouse-grown tomatoes (Ibid, 5).

Zinc, Zn: Zinc is essential for metabolism of nutrients in tomatoes. Moreover, in similarity with copper, zinc also show more tendency for deficiency in soilless medium and in greenhouse (Ibid, 5).

(28)

Molybdenum, Mo: Molybdenum is a constituent of the utilization of nitrogen. Deficiency can occur when soils with low pH, peats, and soilless compost (Ibid, 5).

However, since the amount of nutrients varies with the soil type and environmental conditions the desirable levels of nutrients in table 2.13 is not definitive, only a guideline (Ibid, 8).

Table 2.13 Desirable levels of nutrients for tomatoes per kg of soil and plant, respectively (Sainju et al. 2003, 6).

Nutrients Soil [mg*kg-1] Plant [mg*kg-1]

P 60-70 4000 K 600-700 60000 Mg 350-700 5000 Ca 1000 12500 N 50-100 30000-50000 B 1.5-2.5 40-60 Mn 5-20 30 pH [no unit] 6.5-7.5 - Salt [mmho/cm] 80-100 -

The nutrients that plants need can come from different sources, usually from artificial fertilizers or more organic fertilizers, e.g. municipal solid waste (MSW). The utilization of MSW which means composting household waste and other waste that is similar to household waste that is capable of undergoing aerobic or anaerobic decomposition results in less total amount of landfill waste and a nutritious product that can be used for horticulture. Moreover, the outcome of MSW might contain a different composition compared to artificial fertilizers, by containing toxicity of heavy metals, increased salt content and different ratio between the essential nutrients such as N, K and P. However, the quality of the composition can be adjusted by proper sorting in the origin (Martinez-Blanco et al. 2009, 340).

The composting process can be divided in four main phases.

● The first stage is the pre-treatment. The material, both MSW and pruning waste aggregates and grinds to a maximum size of 80 mm.

● The second stage is composting or decomposition. The grinded matter puts in a tunnel with forced aeration and irrigation system for at least a minimum of two weeks. To maintain a good hygienization of the substance the temperature in the tunnel needs to exceed 65°C for two or three days.

● The third stage is curing. The decomposed material is placed in piles and occasionally turned to enable ventilation. To increase the humidity, the material is moisturized. The process takes approximately eight weeks.

(29)

The total time for the whole process until the compost is able to be applied on the soil without any problems is estimated to around 10 weeks. Decomposing biodegradable materials induces exhaust gases, however since the carbon cycle is short term it results in a neutral CO2 emission (Ibid, 344).

Utilization of compost as a fertilizer for tomato plants does not affect the harvest nor the quality of the tomato.

Biogas slurry, BS, are a by-product from the biogas production where organic compound has decomposed and induced a usable organic fertilizer. The utilization is simple. It is either sprayed directly or the seed is submerged in the substance to stimulate growth and germination (Yu F-B et al. 2009, 262). The fertilizer is rich in nitrogen, phosphorus and potassium, and is in the form of available nutrients. The BS also contains a lot of other necessary nutrients, e.g. sodium and calcium, and other essential elements for the plants such as various kinds of amino acids, vitamins and proteins. Improved soil fertility, and reduced emissions and fertilizer cost is an outcome of long-term utilization of biogas slurry (Yang 2011, 1 959-1 960). Furthermore, utilization of slurry results in better germination of the seed, inhibits diseases, and increases the fruit quality and yield (Yu F-B et al. 2009, 262).

However, with conventional evaporation technology a refined product is possible to obtain, a concentrated biogas slurry (CBS) product that has enhanced nutrient content. With CBS about 10 times the concentration of the main nutrients, such as N, P and K, can be achieved (Ibid, 263). Nutrient content of BS and CBS are presented in table 2.14.

Table 2.14 Concentration of nutrients in BS and CBS (Yu F-B et al. 2009, 263).

Nutrient/mineral Biogas slurry Concentrated biogas slurry

(30)

2.3.4 Water requirement

The main factors that affect the water demand for the plants is transpiration, evaporation and the amount of remaining water in the plant (Schwarz, D et al., 2014, 11). In the paper “Advanced

aquaponics: Evaluation of intensive tomato production in aquaponics vs. conventional hydroponics”

written by Suhl, J et al. (2016) an experimental attempt to reuse wastewater from fish tanks in tomato cultivation was made. The experiment calculated that the plant density would be 2.3 plants per square meter (Suhl, J et al., 2016). However, in another paper “PRODUCTIVE PERFORMANCE OF F-1 TOMATO HYBRIDS AND THEIR F-2 POPULATIONS” (2013) written by Magaña-Lira, N et al. the plant density when cultivating tomatoes in a greenhouse with hydroponic technique is between 2.5-3 plants per square meter (Magaña-Lira, N et al., 2013, 372). Furthermore, in the paper

“Population density and nitrogen fertility effects on tomato growth and yield” written by Whale, EA & Masiunas, JB (2003) a plant density of 3.2 per square meter average a fruit yield of 5.29 and 4.24 kg/plant, while a plant density of 4.2 averaged 4.17 and 3.31 kg/plant (Whale, EA & Masiunas, JB, 2003).

Moreover, a study made in Ontario, Canada found that the water consumption where 28 liters per kg produced tomato for cultivation in a greenhouse, while the water consumption for tomatoes cultivated in the field corresponded to 50 liters per kg produced tomato (Dias et al. 2016, 838).

2.3.5 Humidity requirement

The optimal relative humidity of the air for cultivating tomato plants is between 65 and 75 %, which is the optimal level for the whole cultivation process. During colder season the relative humidity tends to drop to low values due to the condensation of water vapour on the colder cover. When the relative humidity drops below 30% the plants will still grow, but not at optimal conditions. However, with too high relative humidity e.g. more than 85% the fruit growth may be inhibited (Schwarz et al. 2014, 9).

2.3.6 Carbon dioxide requirement

One of the components required for photosynthesis is carbon dioxide (CO2). The average atmospheric

carbon level was measured to be 405 ppm (parts per million) in February of 2017 (NASA, 2017). Plants and vegetation growing in an open environment are affected by the atmospheric condition and thus its CO2 level, however the concentration of CO2 differs locally above vegetation depending on

photosynthesis activity. Plants are autotrophic, meaning that they are able to convert simple molecular compounds and minerals to complex molecules such as carbohydrates, lipids and proteins. They have two fundamental means of supplying energy to themselves; through the processes of respiration and photosynthesis. Respiration is constantly active in plants and is commonly summarized as the process of converting internal glucose (C6H12O6) and atmospheric oxygen (O2) into CO2 and water (H2O)

(reaction 2.2). The process of photosynthesis requires light, CO2 and H2O, resulting in the production

of O2 and C6H12O6 (reaction 2.3) (Holding & Streich 2013, 5-6):

(2.2) C6H12O6 + 6O2 → 6CO2 + 6H2O

(31)

The greenhouse is a structure that protects indoor vegetation from the outside environment and with minimal atmospheric exchange between indoor and outdoor air issues may arise concerning the carbon dioxide level indoors. When there is light the CO2 level can decrease to levels beneath 300 ppm inside

the greenhouse, prompting a growth rate reduction of 40 % or more. It is thus desirable to artificially enrich the air with CO2 during times of light exposure, in order to achieve efficient growth conditions

(Bergstrand et al., 2015). A growth environment with CO2 levels of 800 ppm results in increased yields

of around 30 % (Ibid; Peet & Welles 2005, 289). It is further suggested that with an increased yield of 30 % due to CO2 enrichment, the lighting can be reduced by 30 % accordingly. This way CO2

enrichment could be considered for making energy savings by reducing electricity need for lamps (Bergstrand et al., 2015). CO2 requirements for various plant types, including tomato, are presented in

table 2.15.

Table 2.15 Absorption of CO2 in grams per plant, for four different plant types (Carvajal, 2008).

Plant Absorbed CO2 [g/plant]

Tomato 1 590

Pepper 1 029

Romaine salad 129.8 Cauliflower 342.5

With the use of natural ventilation, through openings in the greenhouse structure, even low wind velocities often enable sufficient exchange of CO2 between the greenhouse and the outside environment.

During such conditions it is inefficient to supply CO2 artificially, due to the high exchange rate of air.

Instead the indoor CO2 level will become similar to the outdoor level of around 400 ppm, which suffice

for crop growth. For more enclosed conditions, however, the cost efficient level of CO2 inside the

greenhouse is suggested to be slightly above 400 ppm, but no more than 1 000 ppm, since economic effectiveness flattens beyond that concentration level (AGA, n.d.). However, if there is little economical restraint for CO2 enrichment, as with biogas combustion, the concentration level may thus be set to

exceed 1 000 ppm. CO2 concentrations of around 2 000 ppm in tomato cultivation have shown increased

starch content in tomato plant leaves and deteriorated photosynthesis as a result, and should thus be avoided (Madsen, 1974).

One study on a greenhouse operation utilizing landfill biogas concluded that higher crop yield from CO2 supplementation (from biogas combustion) was the major contribution to the greenhouse economy

- more so than reduction of heating costs brought by combusting biogas (Jaffrin et al., 2003). Estimated CO2 supply rates for two different target greenhouse CO2 concentrations are shown in table 2.16, where

(32)

Table 2.16 Estimated CO2 supply rates in g/(m2*h) for two target concentrations of CO2, with values for

non-vented and sparsely non-vented greenhouses, respectively (Nederhoff 2004, 53).

Target CO2 concentration

[ppm]

Supply rate without venting

(only infiltration) [g/(m2*h)] Supply rate with sparse venting [g/(m2*h)]

500 5-9 7-13

900 13-31 19-58+

One further aspect of CO2 concentration is the one concerning human safety. A typical threshold for

CO2 concentrations in structures is 1 000 ppm, which may be temporarily exceeded. The hygienic long

term threshold for CO2 concentration lies around 5 000 ppm, a level that will cause damage to human

beings (Claesson, 2016; Arbetsmiljöverket, 2015).

2.3.7 Light requirement

For a plant to engage in the process of photosynthesis and thereby grow it requires exposure to light. Light is thus an important parameter to include in the planning of horticulture. The unit used for measurement of light in plant related sciences is micromoles per square meter and second (µmol/(m2*s)), which accounts for the photosynthetically active radiation (PAR) spectra of wavelength

400-700 nm. Values for conditions of positive net growth begin at 10-30 µmol/(m2*s) and typical levels

during a sunny summer day are around 2 000 µmol/(m2*s). In total, most plants require 12-25 moles of

this light per day (Bergstrand 2015, 4-5). For tomatoes and most other plants the optimal wavelength for growth is 630 nm, which is the wavelength of orange-red light (table 2.17) (Bergstrand et al., 2015). Tomato is an example of a photoperiod sensible crop. That means that it may enter a state of chlorosis, defined by insufficient chlorophyll production, upon intense light exposure for over 18 hours per day (Maslak 2015, 28).

Table 2.17 Photosynthetic response of tomato plant. The photosynthetic response is defined as the uptake of CO2 in µmol/m2/s while exposed to light at 100 µmol/(m2*s) (Bergstrand et al., 2015).

Light, wavelength Blue light, 450 [nm] Green light, 530 [nm] Orange light, 630 [nm] Red light, 660 [nm] Photosynthetic response [µmol/(m2*s)] 2.75 3 4 3.2

During winter months the influx of PAR in Scandinavia is only 1-5 moles/day, meaning that for year round growth additional light sources are required. These are fundamentally in form of lamps and their effectivity in this regard can be measured by the amount of micromoles emitted per watt (µmol/W). This value is usually around 1.5-2 µmol/W (Bergstrand 2015, 5).

(33)

type has been 400 W, but recently 600 W has become more implemented in newer installations. The HPS lamp has an estimated life span of 12 000 - 14 000 hours, however the reflectors usually become partly covered in burnt dust, meaning that occasional cleaning is required for maintained efficient delivery of light (Ibid, 10-11).

LED lights became efficient enough to use for artificial lighting after the introduction of brighter diodes in the 21th century. The diode consists of semiconductor elements with varying energy levels. When a current flows through the diode from a higher to a lower energy level the difference in energy levels is gives off as light. This means that LEDs can be designed to specifically emit desired wavelengths of light, by adjusting the energy levels of semiconductor elements accordingly (Ibid, 14). LED-lights can have a lifetime of up to 100 000 h, so if a daily use of 14 h is assumed, LEDs can be expected to be used for 19 years (Cuce et al. 2016, 50). For LEDs in horticulture it is suggested that a lighting system contains lights of different wavelengths. There should be a large amount of light in the red spectrum, 630-660 nm, and with that some 10 % blue light and 10 % green light is recommended. One alternative is to use white LEDs with a warm color tone, which contains red light to a large amount (Bergstrand et al., 2015). A comparison of HPS and LED lamps are shown in table 2.18.

Table 2.18 A comparison of HPS (High Pressure Sodium) lamps and LED (Light Emitting Diode) lamps for greenhouse horticulture. Three HPS and four LED lamps were evaluated for their efficiency in µmol/W and their special benefits (Bergstrand 2015, 10-11, 14-15, 18).

Type of lamp Efficiency [µmol/W]

Special benefits Heat power generation

HPS 1.5 - 2.0 - Emits light that is of a highly beneficial spectrum for growth - Well tested method for artificial

horticulture lighting

~ 70 % of lamp wattage

LED 1.6 - 2.4 - Ability to manipulate emission spectra

- High efficiency in µmol/W

~ 50 % of lamp wattage

Values for solar irradiation in heat applications are often featured in W/m2, whereas values for it in horticultural applications are in µmol/(m2*s). Thus the ability to convert solar irradiation between these two units is beneficial. For PAR in natural daylight conditions equation 2.1 is suggested for approximate conversion, where solar radiation is in W/m2, the constant 4.57 in µmol/(W*s) and PAR in µmol/(m2*s)

(Environmental Growth Chambers, 2017):

(34)

3 Method and model

Initially in the work process a model of the system is made with a material balance calculation aimed to reuse the products in the cycle and reduce the utilization of resources. Furthermore, the purpose of the model is to give an illustrative view of what is supposed to be achieved, as expressed in section “Purpose and Goals”. The output energy from the biogas facility was set to a fixed value that was obtained from Gunnar Bech, and the facility is dimensioned for small-scale production. With the knowledge of the biogas production and data about the climate where the system is located the energy and mass balance of the system can be defined, using knowledge acquired from the degree program Energy and Environment at KTH. Primarily, the minimum requirements are calculated to keep the system at a sustainable level, and then further optimizations are made.

Both the material and energy balances are made with aid from the data obtained from the literature study. Further, the energy balance is calculated by using an analytic mathematical model.

3.1 Planning the cycle of the system

By demarcating the most essential components a closed cycle of the system is able to be achieved and additionally fulfil the pre-set goals. The system consists of a biogas facility (including a biogas slurry tank), greenhouse and a farm, as seen in figure 3.1, which are included to enable energy and mass flow from one point to another.

(35)

3.2 Dimensions of the greenhouse

As for the type of greenhouse and orientation, there are two types of greenhouse structures that stand out as particularly beneficial: arched roof type and saw tooth roof type. Both types are well suited due to their high level of transmittance in summer, but especially as well during winter (table 2.6), which can be very cold in the set geographical context and thus can aid in reducing the need for additional heating and lighting. The arch type greenhouse has good endurance against wind, satisfying ventilation possibilities and low complexity of construction due to the availability of prefabricated parts and materials (Ponce et al. 2014, 54), thus making it an attractive choice for this model. However, it does not offer as high level of transmittance as the glass covered sawtooth greenhouse. A comparison of the environmental impact of PE-film and glass as greenhouse claddings is desirable to investigate. It is thus seen of interest to simulate energy and material balances for a PE-film covered arched greenhouse and a glass covered sawtooth greenhouse, respectively.

Normally, one-span greenhouses have widths of around 8 meters and lengths of around 16 meters (von Zabeltitz 2011, 221) and thus those are the base dimensions for both greenhouse types analyzed in this model. The height is set to 2.5 meters for the bottom rectangular compartment and 1.5 meters for the arched roof and sawtooth roof compartment, respectively, totalling a maximum height of 4 meters, to accommodate for the greenhouse tomato crops. Illustrations of the arched and sawtooth greenhouse structures are presented in figures 3.2 and 3.3. Both greenhouse types have four troughs for hydroponic cultivation, each of them 14 m long and 1 m wide, with 0.8 m spacing between them. The plant spacing in the troughs is the same as in figure 2.2 (section 2.3.2), meaning that a total of 216 tomato plants are supported. Further greenhouse specifications are presented in tables 3.1 and 3.2.

(36)

Table 3.1 Arched greenhouse structural properties.

Base area: 128 m2

Surface area: 270 m2

Internal volume: 402 m3

PE-film thickness (2 layers): 3 mm per layer Thickness of air layer between PE-films: 4 mm

Figure 3.3. An illustration of the sawtooth greenhouse structure.

Table 3.2 Sawtooth greenhouse structural properties.

Base area: 128 m2

Surface area: 275 m2

Internal volume: 416 m3

References

Related documents

Further the following questions will be answered: Which size does an aquaponics system need to have to supply enough raw materials for the biogas digester and at which local

Industrial Emissions Directive, supplemented by horizontal legislation (e.g., Framework Directives on Waste and Water, Emissions Trading System, etc) and guidance on operating

We also discuss the equivalence of using a Monte Carlo approach - suited for inclusion in neutrino telescope Monte Carlos - and the density matrix formalism.. Identification of

Cost of faults divided with market value excluding installation costs for total heat pump market with second alternative model, year 2008-2013 Figure 21. Cost of faults divided

46 Konkreta exempel skulle kunna vara främjandeinsatser för affärsänglar/affärsängelnätverk, skapa arenor där aktörer från utbuds- och efterfrågesidan kan mötas eller

Both Brazil and Sweden have made bilateral cooperation in areas of technology and innovation a top priority. It has been formalized in a series of agreements and made explicit

The increasing availability of data and attention to services has increased the understanding of the contribution of services to innovation and productivity in

I dag uppgår denna del av befolkningen till knappt 4 200 personer och år 2030 beräknas det finnas drygt 4 800 personer i Gällivare kommun som är 65 år eller äldre i