System Integration of PV/T Collectors in Solar Cooling Systems

(1)

Master Level Thesis

European Solar Engineering School No.198, August 2015

System Integration of PV/T Collectors in Solar Cooling

Systems

Master thesis 15 hp, 2015 Solar Energy Engineering Author:

Arash Ghaghazanian Supervisor:

Chris Bales Examiner:

Ewa Wäckelgård Course Code: MÖ3031 Examination date: 2015-08-27

Dalarna University Energy and Environmental

Technology

(2)

(3)

Abstract

The demand for cooling and air-conditioning of building is increasingly ever growing. This increase is mostly due to population and economic growth in developing countries, and also desire for a higher quality of thermal comfort. Increase in the use of conventional cooling systems results in larger carbon footprint and more greenhouse gases considering their higher electricity consumption, and it occasionally creates peaks in electricity demand from power supply grid. Solar energy as a renewable energy source is an alternative to drive the cooling machines since the cooling load is generally high when solar radiation is high.

This thesis examines the performance of PV/T solar collector manufactured by Solarus company in a solar cooling system for an office building in Dubai, New Delhi, Los Angeles and Cape Town. The study is carried out by analyzing climate data and the requirements for thermal comfort in office buildings. Cooling systems strongly depend on weather conditions and local climate. Cooling load of buildings depend on many parameters such as ambient temperature, indoor comfort temperature, solar gain to the building and internal gains including; number of occupant and electrical devices.

The simulations were carried out by selecting a suitable thermally driven chiller and modeling it with PV/T solar collector in Polysun software. Fractional primary energy saving and solar fraction were introduced as key figures of the project to evaluate the performance of cooling system. Several parametric studies and simulations were determined according to PV/T aperture area and hot water storage tank volume.

The fractional primary energy saving analysis revealed that thermally driven chillers, particularly adsorption chillers are not suitable to be utilizing in small size of solar cooling systems in hot and tropic climates such as Dubai and New Delhi. Adsorption chillers require more thermal energy to meet the cooling load in hot and dry climates. The adsorption chillers operate in their full capacity and in higher coefficient of performance when they run in a moderate climate since they can properly reject the exhaust heat. The simulation results also indicated that PV/T solar collector have higher efficiency in warmer climates, however it requires a larger size of PV/T collectors to supply the thermally driven chillers for providing cooling in hot climates. Therefore using an electrical chiller as backup gives much better results in terms of primary energy savings, since PV/T electrical production also can be used for backup electrical chiller in a net metering mechanism.

(4)

Acknowledgment

For the completion of this Master thesis, the author would like to show gratitude to:

 Dr. Chris Bales, Associate Professor in Energy and Environmental Technology department at Dalarna University for his technical advices, precious guidance and patience in this thesis work and also for his exciting lecture in the solar thermal energy course.

 Eng. Joao Santos Leite Cima Gomes, Assem Sayed and Sathish Kumar Srinivasan Solar Engineers at Solarus AB in Gävle, Sweden for their technical support.

 Eng. Klaus Lorenz, Research Engineer at in Energy and Environmental Technology department at Dalarna University for his technical support in Polysun.

 Dr. Frank Fiedler, Solar Energy Program Director in Energy and Environmental Technology department for his help and support during my studies at Dalarna University, Borlänge, Sweden.

(5)

Nomenclature

ASHRAE American Society of Heating, Refrigerating and Air Conditioning Engineers COP Coefficient of Performance

CPC Compound Parabolic Concentrator EER Energy Efficiency Ratio

Mtoe Mega tonne of oil equivalent PJ Peta Joule

PV/T Photo Voltaic Thermal

(8)

1 Introduction

The International Energy Agency (IEA) has estimated that world primary energy demand increases by 35% between 2010 and 2035 (IEA, 2012), which is 2.1% per year on average (see Figure 1.1).

This increase is due to population and economic growth in developing countries. The increasing global energy demand results more carbon footprints and greenhouse gases which leads to accelerate the global warming.

Figure 1.1. World primary energy demand (according to IEA, World Energy Outlook 2012)

According to the International Institute of Refrigeration (IIR), approximately 15% of all electricity produced worldwide is used for refrigeration and air-conditioning processes (Coulomb, 2006) while around 70% of electricity consumption in Arab states comes from the use of air-conditioning and ventilation devices (IEA, 2012). In addition, a study from the Netherlands Environmental Assessment Agency (Isaac and Vuuren, 2009) shows that global energy demand for the residential heating sector is predicted to increase until 2030 and then stabilize while energy demand for residential air-conditioning is estimated to greatly flourish over the whole 2000–2100 period, mostly influenced by income growth in developing countries which is slightly reinforced by the changing climate (see Figures 1.2 and 1.3).

Figure 1.2. Estimated regional residential energy demand for air-conditioning (according to Isaac and Vuuren, 2009).

0 5000 10000 15000 20000

1990 1999 2008 2017 2026 2035

Mtoe

0 2000 4000 6000 8000 10000 12000 14000 16000 18000

USA Africa Western Europe

Russia India China Rest of Asia

Air conditioning energy demand [PJ]

2000 2050 2100

(9)

Figure 1.3. Estimated regional residential energy demand for heating (according to Isaac and Vuuren, 2009).

Increase in the use of cooling systems is creating peaks in electricity demand and it will lead to more emission. An exciting alternative to reduce these negative effects is expansion of the renewable energy usages. Solar cooling technology can provide an effective solution to reduce the peak electricity consumption as the cooling load is generally high when solar radiation is high.

Solar Heating and Cooling Programme of the International Energy Agency (IEA-SHC, 2014) reported that the solar cooling installation has been growing rapidly from around 60 unit systems in 2004 to 1050 unit systems in 2013 worldwide (see Figure 1.4). Approximately 80% of the solar cooling installations worldwide are installed in Europe. Most of these solar cooling deployments are in Spain, Italy and Germany.

Figgure 1.4. Market development 2004–2013 of small to large-scale solar air conditioning and cooling systems (according to IEA-SHC, 2014).

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

USA Africa Western Europe

Russia India China Rest of Asia

Heating energy demand [PJ]

2000 2050 2100

0 200 400 600 800 1000 1200

2004 2007 2009 2011 2013

Number of solar cooling installations

Europe World

(10)

1.1 Aims and Scope

The main objective of this thesis work is to determine quantitative energy production of PV/T collectors for solar cooling applications. This work will more specifically look into the performance of PV/T collectors produced by Solarus company and will estimate their energy yield in a complete designed solar cooling system. The study case is a one floor governmental building with a dimension of 15 m in length, 8 m in width, and 3 m in height. The simulation model is defined to operate at time between 8 am and 6 pm, and represents a low energy office building in North–South orientation.

A parametric simulation study investigates the potential of different type of solar cooling strategies and it aims to select a suitable solar cooling system with regards to Solarus PV/T collectors and building thermal comfort in four different climates: Dubai, New Delhi, Cape Town and Los Angeles.

1.2 Method

In order to accomplish the thesis goals, the following steps require to be taken:

1- Literature review in PV/T collectors, thermal and photovoltaic systems, and solar cooling methods and technologies.

2- Selecting a suitable solar cooling system according to thesis aims and Solarus PV/T collector properties.

3- Design a solar cooling system according to PV/T technical data provided by Solarus company and model it with Polysun software.

4- Estimate the building cooling load in first iteration and then sizing the system components in second iteration.

5- Choice and definition of key figures for the project.

6- Evaluate the cooling energy production of the model in the four different locations with variation of collector and hot water storage size.

(11)

2 Solar cooling system requirements

2.1 Parameters of indoor thermal comfort

The American Society of Heating, Refrigerating and Air Conditioning Engineers defined thermal comfort as the state of mind that expresses satisfaction with the surrounding environment (ASHRAE Standard 55, 2004).

Figure 2.1. Elements that effect to the human thermal comfort (according to Solair, 2009).

As it can be seen from Figure 2.1, thermal comfort is affected by solar radiation, heat conduction and convection to the building shell, as well as evaporative heat loss in terms of humidity. Thermal comfort in indoor environment is achieved when the heat generated by human metabolism is allowed to dissipate with the surroundings. Therefore any heat gain or loss beyond this can appear as sensation of feeling hot or cold.

There are several standards and models that recommend indoor environment comfort based on ambient temperature, relative humidity, occupant clothing, activity level and type of building. The European standard EN 15251 defines two comfort ranges (minimum room temperature in winter, maximum room temperature in summer) for design and dimensioning of the building thermal comfort and air conditioning systems. Assuming different criteria for the activity level and building type, different categories of the indoor environment are established (see Tables 2.1 and 2.2).

Table 2.1. Description of the applicability of the categories used (EN 15251, 2007).

Category Explanation

1 High level of expectation and is recommended for spaces occupied by very sensitive and fragile persons with special requirements.

2 Normal level of expectation and should be used for new buildings and renovations 3 An acceptable, moderate level of expectation and may be used for existing buildings 4 Values outside the criteria for the above categories

(12)

Table 2.2. Examples of recommended design values of the indoor temperature for design of office buildings and HVAC systems. (EN 15251, 2007).

Type of building/space Category Minimum temperature

requirement for heating [°C] Maximum temperature requirement for cooling[°C]

Office (cellular and open

office)

1 21 25

2 20 26

3 19 27

The American standard, ASHRAE 55, addresses the adaptive approach to indoor thermal comfort.

The ASHRAE mathematical model result is based on occupant’s activity levels, clothing insulation, air temperature, metabolic rates and occupant acceptability under real working conditions. Figure 2.2 illustrates the comfort zone for environments that meet the above criteria and where the air speed is less than 0.20 m/s.

Figure 2.2. Acceptable range of operative temperature and humidity (according to ASHRAE 55, 2004).

2.2 Cooling technologies

This section dedicated to study different cooling technologies that are feasible for coupling with a solar thermal energy source. The most common cooling systems are desiccant evaporative cooling, absorption chillers, and adsorption chillers. The vapor compression machine is also discussed since they are used as a reference system.

2.2.1. Absorption chiller

Absorption chillers use a continuous cycle process based on two different substances, a refrigerant and a liquid absorbent. Absorption chillers usually use water as the refrigerant and lithium bromide or lithium chloride as the absorbent. The whole process is based on different vapor pressure of these two substances. Cold is produced when refrigerant is periodically interacted (absorbed and released) by absorbent which leads to evaporation and absorption. Absorption process may be single or double effect, classified by regeneration steps of the refrigerant and the absorbent. The driven

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

10 13 16 19 22 25 28 31 34 37

Humidity ratio

Operative temperature [°C]

Humidity - 10 [%]

Humidity - 30 [%]

Humidity - 50 [%]

Humidity - 70 [%]

Humidity - 90 [%]

(13)

heat required at a temperature between 80-95 °C for single effect chillers and about 180 °C for double effect chillers.

2.2.2. Adsorption chiller

Adsorption process is a surface phenomenon. In adsorption chillers the liquid refrigerant is adsorbed onto a solid sorbent (silica gel or zeolite) while proving cold through the evaporation. The refrigerant continues to be adsorbed until the saturation of the solid sorbent, and then the process is reversed. There is heat requirement for desorption of stored water from the sorbent. The required driving heating temperature to the generator depends on materials, for example a temperature between 55 and 90 °C is required for silica-gel.

2.2.3. Desiccant evaporative cooling (DEC)

Desiccant evaporative cooling is considered an open cycle process which is directly used for conditioning the air. The DEC system is based on dehumidification of ambient air by using hygroscopic materials such as zeolite or silica gel. The evaporation of water will reduce the ambient air temperature to the desired level. The required heat depends on cooling load and environment conditions, and temperature is in range of 50-100 °C. Desiccant evaporative cooling systems have some technical limitations, particularly working in hot and humid climates, because of rusting, warping, and mildew of susceptible materials. In addition, they may require more maintenance due to their rotating elements.

2.2.4. Conventional vapor compression

Most of the cold production for building air-conditioning is generated with conventional vapor compression. The refrigeration cycle is based on compression and expansion of the refrigerant vapor to extracting heat from liquid to the gas phase. The evaporation provides cold and the closed cycle continues when the refrigerant is compressed, expanded, and evaporated again in a cyclic process. This process requires power to drive the electrical compressor.

2.2.5. Performance comparison between conventional compression and thermally driven chillers

Any cooling system consumes energy (thermal or electrical) to extract the heat from conditioned space and reject it to the outdoor environment with supplied heat (see Figure 2.3). A key figure to characterize the energy performance of cooling machine is the Coefficient of Performance. For thermally driven refrigeration systems the defined as follows:

Equ. 2.1

For the conventional vapor compression machine, the known as Energy Efficiency Ratio (EER) which is defined as follows:

Equ. 2.2

(14)

Equations 2.1 and 2.2 define ratio of cold energy obtained to energy is supplied to the system.

Figure 2.3. Schematic diagram of energy flows in compression (left) and thermally driven (right) refrigeration cycle.

Table 2.3 compares the performance data of commercially available chillers.

Table 2.3. Overview of sorption and compression technologies (according to Kohlenbach, P. and Jakob, U. 2014)

Parameter

Absorption

Adsorption DEC Compression

Single effect Double

effect Single effect

Refrigerant Water Water Ammonia Water - Freon, …

Sorbent Lithium bromide

Lithium

bromide Water Silica gel Lithium bromide-

Silica gel -

Cooling medium Water Water Water-glycol Water Air Air-Water

Hot water

temperature [°C] 80 - 95 140 -190 75 - 100 55 - 95 50 - 100 - Cooling water

temperature [°C] 20 - 40 20 - 40 20 - 40 20 - 45 - -

Chilled water

temperature [°C] 6 - 20 6 - 20 -30 - 20 6 - 20 16 - 20 -

COP 0.6 – 0.8 1.1 – 1.35 0.5 – 0.7 0.5 – 0.65 0.5 - 1 2 - 5

2.3 Heat rejection systems

A heat rejection system or re-cooler is a system that extracts the absorbing heat from absorber and rejects heat to the ambient. This heat is sum of heat removed from chilled area and driving heat to the chiller. The climate conditions play an important role for selecting an appropriate heat rejection system. The re-coolers are characterized by their cooling medium as following.

2.3.1. Open wet re-cooler

An example of open wet re-cooler system is shown in Figure 2.4. This system consists of several nozzles which distribute the cooling water onto trickle packing. The water droplets can be cooled down by blowing fresh air through the sides of package’s shell and then accumulates into a basin with the rest of condensed vapors. The accumulated water in the basin is pumped back to the chiller and this circle continues. The main advantage of wet re-cooling system is low temperature level of

(15)

cooled water which can be lower than ambient temperature, but they require more investment cost.

On the other hand, the wet re-cooling system consumes more water and has higher maintenance cost. In general wet re-cooling systems occupy more space and they have legionella risk.

Figure 2.4. Open wet re-cooling system (according to Henning et al. 2013).

2.3.2. Closed wet re-cooler

In the closed wet re-cooling system, water is cooled down in pipes when it passes through where water is distributed. As it can be seen in the Figure 2.5, the hot water from heat resource has different circulation loop then distributed water. This is main advantage of closed wet re-cooling because it avoids any risk of mixing with dirt and out-doors dust. These biological materials can foul in pipes and reduce the heat transfers. The closed wet re-cooler cannot decrease cooled water temperature level as open wet re-cooler does. This system has lower operational cost but it has higher investment cost due to complex design and more components.

Figure 2.5. Closed wet re-cooling system (according to Henning et al. 2013).

2.3.3. Dry re-cooler

Dry re-cooler uses finned heat exchanger (air to water) to cool down the water coming from the heat source. This heat exchanger rejects the heat from water to the ambient air (see Figure 2.6). The cooling temperature level is based on capacity of fan and also ambient dry bulb temperature. Dry re- coolers cannot achieve a temperature below the ambient dry bulb temperature. This system also has higher electrical energy consumption, but lower maintenance cost.

(16)

Figure 2.6. Dry re-cooling system (according to Henning et al. 2013).

2.3.4. Hybrid re-cooler

The hybrid re-cooling system relies on dry re-cooling and evaporating cooling concept. This system consists of cooling water pump which circulates the cooling water in a close circuit with exchanger.

In a cool climate the hybrid re-cooling system is mostly like a dry re-cooler while at hot weather conditions it operates as wet re-cooler and the exchanger is cooled down by water from the basin and also fresh air blown through cases (see Figure 2.7). The main advantage of the hybrid re-cooler is its lower cooling temperature compare to the dry re-cooler and it is below the dry bulb temperature. However, the hybrid re-cooler has higher water consumption, higher investment cost and also requires more maintenance due to water circulation.

Figure 2.7. Hybrid re-cooling system (according to Henning et al. 2013).

2.4 Fan coil

Fan coil unit is a device that consists of a heat exchanger and fan. The fan circulates indoor air through the heat exchanger which is supplied with hot or chilled water. The heat exchanger transfers heat or cold to the air drawn by fan from the conditioning area. The fan coils are classified by their number of pipe systems. A two pipe system operates with one pipe for supply form heat source to heat exchanger and one pipe for returning the medium to heat source. Two pipe systems can be used for both heating and cooling depending on application. However, a four pipe system has separate coil and piping for heating and cooling applications, therefore these units are able operate in different seasons and weather conditions. A sketch of simple fan coil is illustrated in Figure 2.8.

(17)

Figure 2.8. Sketch of a typical fan coil (according to Henning et al. 2013).

The main advantages of using fan coils to distribute heat or cold for the building are their simplicities. They occupy less space and just require piping installation. They also allow to distribute air flow in the building individually which helps to reduce the energy consumption. In the other hand the fan coil systems require additional fresh air ventilation and also remove the condensate from units.

2.5 Hot water storage

The hot water storage tank is one of the key elements in solar heating and cooling systems. The hot water storage may minimize the use of backup system while overcoming the gaps in solar thermal power availability. In addition, the hot water tank stores the heat at the appropriate temperature levels and does the stratification to keep hot water and cold water in different layers which also reduces the energy losses.

2.6 Backup heater

In a solar thermal system, the backup energy source (oil, gas, electricity, etc.) supplements the heating demands when solar power is not available. For cooling and air condition applications, the backup heaters are required especially during days with high cooling loads and in hot and humid climates. These auxiliary heaters can provide thermal energy to the cooling system when solar power does not deliver enough heat to meet the load or when the weather becomes cloudy.

2.7 Solar collector

There are different types and designs of solar thermal collector available, namely flat plate collectors, air collectors, evacuated collectors, PV/Thermal collectors, CPC and tracking concentrator collectors. Many of them are possible in cooling applications since most of the thermally driven cooling systems require supplying heat between 50 °C to 250 °C. However, in this section a brief technical data description of PV/T collectors is presented since this study work will only focus on utilizing Solarus company’s PV/T in the particular solar cooling design.

Solar energy can be either utilized for producing electricity directly through the photovoltaic panels or to produce heat by solar thermal collectors. However, combining the photovoltaic cells with a solar thermal collector can simultaneously convert solar radiation into electricity and heat. This type of solar collector is known as PV/T and it produces more energy per unit surface area than side by

(18)

side photovoltaic panels and solar thermal collectors. The PV/T collectors can be used in several applications, but the most suitable are applications that need heat in between 60 °C to 80 °C.

2.7.1. Solarus PV/T collector

Solarus AB is a Swedish company that develops and manufactures PV/T and solar thermal collectors. Their solar collector products have asymmetric Compound Parabolic Concentrator (CPC) configuration which leads to increasing the flux of radiation on receiver. As it can be seen in Figure 2.9, the receiver is located in the side of an asymmetric CPC. The receiver contains photovoltaic cells which are laminated on the both side of the absorber. The fluid runs through the channels prepared inside the absorber and it cools down the solar cells which it leads to improve the electrical efficiency of the PV/T collector.

Figure 2.9. Front view (top) and side view (bottom) of Solarus PV/T module (Solarus AB).

(19)

Table 2.4 shows the characteristic of PV/T solar collector manufactured by Solarus AB.

Table 2.4. Technical parameters of the Solarus PVT collector (according data provided by Solarus AB).

Mechanical

Module dimension [mm] 2.374  1.027  231

Module gross area [m²] 2.4

Module aperture area [m²] 2.17

Maximum operational temperature [°C] 200

Collector weight [kg] 53

Maximum working pressure [bar] 10

Thermal

Zero-loss coefficient [-] 0.785

Heat-loss coefficient [W/m².K] 4.484

Second degree heat-loss coefficient [W/m².K²] 0.0034

Peak thermal power [W] 1500

Electrical

Maximum electrical power at STC [W] 230

Maximum power point voltage [V] 22

Maximum power point current [A] 10.5

Open circuit voltage [V] 22.8

Short circuit current [A] 16.04

(20)

3 System design and selection of solar cooling components

The previous chapter discussed about foundation of solar cooling system requirements and provided wide information about climate considerations, comfort criteria and some available of cooling theologies. This chapter deals with selection criteria of a suitable solar cooling system and its components for the required study locations.

3.1 Solar cooling system topology

A basic topology of solar thermal cooling systems is illustrated in Figure 3.1. The collector converts the solar radiations into the heat and the heat exchanger transfers the heat to the hot water storage tank. An auxiliary heater is considered for the times when solar heat gain is not available or when it is not enough to meet the load. The hot water storage tank is used for balancing heat deliveries between solar and boiler to the chiller. In this system, a thermally driven chiller is used to meet the cooling demand, and then the produced useful cold is supplied to the building via a fan coil. The cold water storage tank stores the cold for the times other than is required. A heat rejector is applied to transfer the removed and supplied heat to the ambient.

Figure 3.1. Basic schematic of a solar cooling system (according to Henning et al. 2013).

3.2 Selection criteria of chiller

This thesis aims to study and design a solar cooling and air conditioning system by using Solarus PV/T collector. Therefore, it is required to choose a cooling technology which is compatible with PV/T characterizations. In order to keep both the electrical and thermal efficiency of PV/T collectors at an acceptable level it is required to operate at low heat temperature (60 – 80 °C). The thermal efficiency of PV/T collector decreases by increasing the fluid temperature inside the collector as it is shown in Figure 3.2.

.

(21)

Figure 3.2. Efficiency of Solarus PV/T collector, calculated at ambient temperature of 25°C and 800 W/m² radiation level.

There are two cooling technologies available to integrate with PV/T and meet its low temperature operation requirement; namely desiccant evaporative cooling system and adsorption system. A desiccant evaporative cooling system operates at temperature range between 50-100 °C; however this system is not suitable in hot and humid climates. This system is less efficient due to rusting and mildew of susceptible materials and needs more maintenance. The second alternative is an adsorption system. Adsorption cooling systems work at lower temperature range between 55–95 °C.

Adsorption chillers have a lower COP and they consume less electricity due to absence of pumps and moving parts. Therefore a suitable cooling technology for this particular study case is an adsorption chiller.

3.3 Selection criteria of heat rejection system

The technical information about re-coolers which presented in chapter two is summarized in Table 3.1. The temperature approach is difference between temperatures of the cooled water returning from re-cooler and wet or dry bulb of the ambient air. The term dry bulb temperature refers to the ambient air temperature and the wet bulb temperature is the lowest temperature that can be reached by the evaporation of water under current ambient conditions. The wet re-coolers can provide lower cooling water compare to the dry re-coolers, however dry re-coolers are more suitable for hot and humid climates since the evaporation of supply water in wet re-coolers is limited in high humid and hot climates. In addition, water consumption of wet re-coolers are very high and it should not be a good option for the climates where there is water shortage. Therefore based on above constraints for wet re-coolers also for the simplicity, a dry re-cooler will be considered for all the study locations.

0 0.2 0.4 0.6 0.8 1

30 50 70 90 110 130 150 170 190

Efficiency [-]

Average Fluid Temperatue [°C]

Thermal Electrical

DEC

Adsorption

Absorption Absorption 2 - effect

(22)

Table 3.1. Comparison between different heat rejection systems (according to Henning et al. 2013).

Parameter Open wet Closed wet Dry Hybrid

Temperature approach Wet bulb 4 – 8 K Wet bulb 4 – 8 K Dry bulb 5 – 9 K Dry bulb 5 – 9 K

Investment costs ^Low ^High ^High ^High

Energy consumption ^Low ^Low ^High ^Low

Water Consumption ^High ^High ^No ^Medium

Hygienic problem ^Yes ^Yes ^No ^Yes

Maintenance ^High ^High ^Low ^High

Required space ^High ^High ^Low ^High

3.4 Selection of cold and hot water storage tanks

The aim of cold water storage is to store the produced chilled water at times other than when cooling is needed. Since the reference office is defined to operate at time between 8 am and 6 pm, then there is no cooling load at nights, therefore a cold water storage tank is not necessary for this particular application. For hot water storage (Henning et al. 2013), the hot water storage tank can be sized based on chiller capacity, coefficient of performance and useful temperature difference (see Figure 3.3).

Figure 3.3. Required storage volume for hot water storage (according to Henning et al. 2013).

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

10 12 14 16 18 20 22 24 26 28 30

Required storage volume [m³ per kWh of cold]

Useful temperature difference [K]

COP 0.5 COP 0.55 COP 0.6 COP 0.65 COP 0.7 COP 0.75 COP 0.8

(23)

4 Polysun model design and experimental studies

This chapter describes the process of performing the solar cooling system in Polysun with its featured simulation library. The chapter begins with climate data and implementation of each unit in Polysun software.

4.1 Polysun simulation software

Polysun simulation software is developed by Vela Solaris Corporation, a spin-off company from the SPF Institut für Solartechnik. Polysun is used to calculate the energy production and consumption of a solar thermal or photovoltaic system (Kohlenbach, P. and Jakob, U. 2014). The program also offers features for performance optimization and easy system sizing. In addition, it includes library with component catalogue database of solar thermal collectors, photovoltaic modules, inverters, heating and cooling units. The user can run a simulation by using one of the sample systems which exist in the program or by designing a new model configuration. The user is also able to add a new component or catalogue data to the library. The Polysun calculations are based on dynamic simulation model by using world statistical weather data. The program generates component and graphical results which can also be used for marketing support.

4.2 Climate data

Weather and radiation data are needed to analyze for designing and sizing of a solar cooling system.

The hourly values of solar irradiation, maximum and minimum ambient temperature, air humidity as well as wind speed are needed to be used in the prediction of the performance of solar cooling systems. These meteorological data usually are available in simulation tools, but the climate data analysis need to be carried out for better understanding of behavior of solar cooling system components.

4.2.1. Climate and weather in Dubai

Dubai is a major city in the United Arab Emirates with latitude 25°26′ N and longitude 55°20′ E.

Weather in Dubai is characterized by a subtropical desert climate with hot and sunny conditions.

Summer in Dubai starts in April with average high temperature of 32.5 °C during the day and lasts until October with temperature climbing to 40 °C. Summers are very hot in Dubai and humidity is high on the coastline where the sea temperatures can reach to 37 °C with humidity above 50%.

Winter in Dubai occurs between January and March; however winters are still warm with average high temperature 23.4 °C and low temperature around 14 °C (see Figure 4.1)

(24)

Figure 4.1. Monthly climate data for Dubai (according to Polysun weather data).

4.2.2. Climate and weather in New Delhi

New Delhi is capital of India with latitude 28°35′ N and longitude 77°12′ E. Delhi has an extreme climate with monsoon and humid subtropical conditions which also classified as semi-arid region.

Summer in Delhi starts in early April with average temperature of 29 °C and the temperature climbs to near 40 °C in May. The monsoon starts in late June and peaks in July (with about 180 mm of rainfall) and lasts until mid of September when average temperature is 29 °C. Winter in Delhi occurs between November and March; with average temperature around 12 °C (see Figure 4.2).

Figure 4.2. Monthly climate data for New Delhi (according to Polysun weather data).

0 50 100 150 200 250

0 10 20 30 40 50 60 70

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Irradiance [kWh/m²]

Temperature [°C], Humidity [%]

Average outdoor temperature Average air humidity Monthly global irradiation

0 50 100 150 200 250

0 10 20 30 40 50 60 70 80

(25)

4.2.3. Climate and weather in Cape Town

Cape Town is the capital and one of the most populous cities in the South Africa with latitude 33°55′ S and longitude 18°25′ E. Weather in Cape Town is characterized by a warm-summer Mediterranean climate. Cape Town's long summer runs from mid November to mid March with average high temperature of 29 °C in February. Winter in Cape Town starts in June and ends in late August with an average temperature of 13 °C, however winter in Cape Town is not essentially cold, and outdoor temperature can still reach to 18 °C (see Figure 4.3).

Figure 4.3. Monthly climate data for Cape Town (according to Polysun weather data).

4.2.4. Climate and weather in Los Angeles

Los Angeles is the second largest city in the United States with latitude 34°06′ N and longitude 118°21′ E. Los Angeles climate is classified as Subtropical Mediterranean with relatively modest transitions in temperature. Summers are hot, dry, and begin in late June, lasting through end of October. Los Angeles maximum temperature reaches 28 °C in August while it falls to 18°C in January. Los Angeles also has plenty of sunshine throughout the year (see Figure 4.4).

0 50 100 150 200 250 300

0 10 20 30 40 50 60 70 80 90

(26)

Figure 4.4. Monthly climate data for Los Angeles (according to Polysun weather data).

4.3 The building model in Polysun

The simulation starts by defining the reference building model in Polysun. The reference office is a free standing, one storey building with total area of 120 m². The building is oriented along the north- south axis. The office is considered a low energy building with overall heat loss coefficient of 0.35 W/K/m². This heat loss coefficient is also known as U value and is taken from the Polysun library.

The U value depends on materials and layer thicknesses of walls, roof and floor of the reference building. The internal loads are the heat gains due to occupation, office electrical equipment and lighting at the office operational time. The operation time of this reference office is 8 am to 6 pm.

The heat dissipation to the surroundings by occupation is based on the number of people are present in the office and their activity level. The rate of heat dissipation from the office equipment and lighting depends how the office facilitated with electrical devices and also personnel presence. In general, the internal heat gains value is given as watt per square meter. The ventilation rate is 0.3 times per hour and infiltration is assumed as 0.3 times per hour due to leakages in the building envelope during office operating time. According to the European standard EN 15251, the necessary cooling set temperature of the reference building is defined to 23 °C for day operations in a landscaped office. The Polysun software doesn’t have feature to set the desire humidity level in the building. There is no internal gains assumed during night time between 8 pm to 8 am and building set temperature defined to 24 °C. The ventilation system is also switched off during the absence of the people. More detailed information about the reference office is provided in the Table 4.1.

0 50 100 150 200 250

0 10 20 30 40 50 60 70 80

(27)

Table 4.1. Building description - Office Building, low energy house (according to Polysun building model)

Parameter Value Unit

Length of building 15 [m]

Width of building 8 [m]

Height of building 3 [m]

Number of floor 1 -

U- value of the Building 0.35 [W/K/m²]

Specific cooling energy demand 160 [kW/h/ m²]

Window to wall area ratio - South 25 [%]

Window to wall area ratio - North 13 [%]

Window to wall area ratio - East 25 [%]

Window to wall area ratio - West 6 [%]

g-value - windows 0.7 -

Air change 0.3 [1/hr]

Air infiltration 0.3 [1/hr]

Internal heat gain - light 15 [W/m²]

Internal heat gain - equipment 10 [W/m²]

Internal heat gain - people 9.4 [W/m²]

Heat capacity of building 750 [kJ/K/ m²]

4.4 Adsorption chiller model in Polysun

The next step is to select a cooling system in Polysun. As discussed in chapter three, a suitable cooling technology is an adsorption chiller for this particular study project. Every adsorption chiller has an individual performance map where there is defined the chiller capacity and coefficient of performance or COP. Both chiller capacity and COP values are depended on supply, cold and chilled water heat temperatures. Figures 4.5 and 4.6 show the performance of ACS08 adsorption chiller which manufactured by SorTech company (Polysun adsorption chiller library). The values are taken from Polysun component library.

Figure 4.5. Capacity chart of ACS08 adsorption chiller at chilled water 18 °C (according to Polysun library).

0 2 4 6 8 10 12 14

20 30 40 50

Capacity (kW)

Cold Water temperature (°C)

HW=65 °C HW=75 °C HW=85 °C HW=95 °C

(28)

Figure 4.6. COP chart of ACS08 adsorption chiller at chilled water 18 °C (according to Polysun library).

4.5 Fan coil system in Polysun

A fan coil is used to deliver the produced cold to the building. For cooling application, the fan coil runs in cooling operation mode. The student version of Polysun offers just a few types of fan coil.

Therefore, user should define its own model and add it to the Polysun library.

4.6 Heat rejection system in Polysun

Polysun library contains two types of heat rejection systems, namely wet re-cooler and dry re-cooler.

For this study project the heat rejection is obtained by means of a dry re-cooler. In Polysun, a dry re- cooler is essentially a fan coil, but it is needed to change its operation mode from cooling to the heating which will dissipate the heat from the chiller. The other parameters of re-cooler are nominal heating power, hot water flow rate, air flow rate and their inlet and outlet temperatures.

4.7 Hot water storage in Polysun

A variety of storage tanks are available in the Polysun library. These storage tanks are from different manufactures and they are in different sizes and qualities. For this study case, a six port buffer tank is selected which is suitable to combine with solar circuit, auxiliary heater and chiller.

4.8 Backup heater in Polysun

Polysun library includes several different types of the boiler, namely gas, electrical, oil, pellets and firewood boilers. A gas boiler is selected for supplementary heating of the hot water loop to the generator.

4.9 System definition and cooling load simulation – 1st iteration

In order to size each component of the solar cooling systems, it is required to know the cooling load of the building in different study climates. Figure 4.7 illustrates the layout of this simulation model in Polysun. The simulation model consists of the following system elements:

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

20 30 40 50

COP

Cold Water temperature (°C)

HW=65 °C HW=75 °C HW=85 °C HW=95 °C

(29)

1) Building model

2) Adsorption chiller which its characterization is described in section 4.4.

3) Dry re-cooler. (72.8 kW) 4) Fan coil. (72.8 kW)

5) Hot water storage tank (1.2 m³) 6) Gas boiler. (20 kW)

7) Solar collector

8) Heat exchanger and pumps 9) Control system units

Figure 4.7. Sketch of cooling system modelled in Polysun software.

Here, the aim is just to estimate the cooling load of the building. Therefore the size of elements in the solar loop is not important at this stage of simulation. The only parameters needs to be considered are the capacity of chiller, boiler, re-cooler and the fan coil. For the beginning of the simulation, a capacity of 20 kW is assumed for the backup heater and a capacity of 72.8 kW is assigned for both dry re-cooler and fan coil. These values are seemed to be overestimated but again;

here the only delivered cold to the building is of interest.

As it is shown in the Figure 4.7, the hot water from the heat exchanger in solar loop is connected to port two and the return is connected to the port one of the hot water tank. The boiler output is connected to port four and the return is connected to port three of the storage tank. The input of the chiller generator is connected to port eight and the return to port five of the storage tank (see Figure 4.8). This strategy of connecting components to the hot water storage tank helps to have stratification in the tank and it also improves the overall efficiency of the system.

(30)

Figure 4.8. Layout of hot water storage tank ports.

4.9.1. Controlling strategies Control of the solar loop

 Measuring outflow temperature of collector and layer three in hot water storage tank.

 Taking the aperture area of collector. Assigning flow rate of 40 l/h/m² for the both pumps.

 Maximum collector temperature and tank respectively are 170 °C and 100 °C.

 The pump in solar loop is switched on when temperature difference of 4 °C is reached.

 The pump in solar loop is switched off when temperature difference of 2 °C is reached Control of the chiller and pump in generator loop

 Measuring the building temperature and the layer ten of storage tank.

 Checking the cooling set point temperature of the building.

 If the building temperature is lower than cooling set point temperature, then cut-off the chiller and the pump in the generator loop.

 If the building temperature is higher than cooling set point temperature, then cut-in the chiller and the pump in the generator loop.

 Setting the generator pump flow rate to 3600 l/h.

Control of the boiler

 Measuring the temperature layer eleven of hot water storage tank.

 Switching on the boiler if the temperature layer eleven is lower than 80 °C.

 Switching off the boiler if temperature layer eleven is greater than 95 °C.

Control of the pump in re-cooler loop

 Measuring the on and off state of chiller. Applying the same to the pump in re-cooler loop.

 Assigning flow rate of 21600 l/h to the pump in re-cooler loop.

Control of the pump in fan coil loop

 Measuring the on and off state of chiller and applies to the fan coil loop pump.

 Assigning flow rate of 3960 l/h to the pump in fan coil loop.

4.10 Pipes, Pumps and heat exchanger

All pipes are copper with internal diameter of 20 mm and external diameter of 22 mm for this pre- simulation. The length of pipes in solar loop is 22 m, in generator circuit is 4 m, in fan coil and re-

(31)

cooler loops are 20 m and 26 m respectively. The insulation material is a mixture of loose glass fiber and mineral wool with thermal conductivity of 0.045 W/m/K and density of 250 kg/m³. Pumps are similar to each other with electrical power of 50 W. The heat exchanger is sized as medium with transfer capacity of 10000 W/K.

4.11 Cooling load simulation – first iteration

The first simulation was undertaken for Dubai climate. But the cooling demand of building didn’t meet by using the selected adsorption chiller and there were many days in July and August months where the building temperature was above the set point temperature at 23 °C. Therefore it was required to replace the chiller with a larger capacity. The adsorption library of the Polysun was very limited in the student version, thus a modified version of used chiller was placed into the cooling model. The modification was done by just increasing the capacity values in the cold water temperatures table inside the performance map of the ACS08 adsorption chiller. The updated performance map of the adsorption chiller has following characterizations (see Figure 4.9)

Figure 4.9. Modified capacity of ACS08 adsorption chiller (according to Polysun library).

4.12 Results of cooling load simulation – second iteration

The second simulation succeeded to cover the building cooling demand in Dubai. The same process was repeated for other climates. In Delhi, the same chiller (see Figure 4.9) is used as Dubai since they have the same climate, however, for Cape Town and Los Angles cooling models it was enough to use a smaller chiller (see Figures 4.5 and 4.6) since they have mild weathers. The following results are obtained after running the Polysun software in the four different study locations (see Table 4.2).

Table 4.2. Annual cooling load and design cooling load of office building in different study locations.

Unit Dubai Delhi Los Angeles Cape Town

Annual delivered

cooling energy [MJ/a] 140000 122000 61000 51000

Design cooling

load, (Date) [kW] 13.45

(July 3) 12.65

(June 22) 10.40

(August 12) 9.85 (February 13)

0 5 10 15 20 25

20 25 30 35 40 45 50

Capacity (kW)

Cold water temperature [°C]

Performance chart at chilled water =18 °C

HW=65 °C HW=75 °C HW=85 °C HW=95 °C

(32)

The first row of the Table 4.2 represents the yearly cooling energy delivered to the building and second row the design cooling load is the cooling capacity to deliver the total 24 hour cooling demand on the hottest day during office hours with constant capacity. The second row values will be used to size the pipes, pumps and calculate the fan coil and re-cooler capacities.

4.13 Fan coil and re-cooler sizing methodology

Estimating the cooling load of the building helps to calculate the size of fan coil and also re-cooler.

These quantities have following relationships:

Equ. 4.1

Equ. 4.2

Where the is delivered cold, the is supplied heat to generator and is the rejected heat to ambient. The air flow rate of the re-cooler system can also be calculated from equation 4.3.

Equ. 4.3

Where and are air density and specific heat capacity of air respectively. is air inlet and outlet temperature difference.

4.14 Sizing pipes and selecting pumps methodology

The size of pipe depends on water flow rate and its acceptable maximum velocity inside the pipe.

This relationship is shown in equation 4.4.

Equ. 4.4

Where the [mm] is internal diameter of pipe, [m/s] is maximum velocity of water and [m³/s]

is water flow rate inside pipe. The acceptable water velocity in pipes for solar applications is less than 0.5 m/s. The water flow rate [l/h] can be calculated by following equation:

Equ. 4.5

Where the [W] is the heat (or cold) energy flow, the [kg/m³] is water density, the [kJ/kg.K] is specific heat capacity of water and the is inlet and outlet temperature difference. Pumps also can be sized based on their flow rate range and power consumptions. For this study project, pumps are picked from the Polysun library according to their maximum flow rates in each loop.

According to equation 4.5 and equations 4.1 and 4.2, the flow rates in each loop can be calculated as following steps:

(33)

 Using equation 4.5 for calculating the flow rate in chilled water loop, chiller capacity is given in the Table 4.2 as daily average for each city. The for chilled water loop can be considered equal to 5 K (according to Solair Guidelines, 2009).

 The supplied heat capacity to the generator can be calculated by using equation 4.1. The chiller cop can be measured from Polysun at operation conditions for each city. The flow rate in generator loop can be estimated by finding the value of supplied heat to the generator and considering equal to 10 K (according to Solair Guidelines, 2009).

 The re-cooler capacity can be calculated using equation 4.2. Once the rejected heat calculated, then the flow rate in re-cooler loop can be calculated from equation 4.5 and considering equal to 7 K (according to Solair Guidelines, 2009).

Table 4.3 shows the design parameters for calculating flow rates, fan foil and re-cooler capacities in different study locations.

Table 4.3. Assumed temperature difference in each loop and COP for different study locations.

Component Unit Dubai Delhi Los Angeles Cape Town

Chiller capacity [kW] 13.45 12.65 10.40 9.85

in chilled water loop [K] 5 5 5 5

in generator loop [K] 10 10 10 10

in re-cooler loop [K] 7 7 7 7

COP in operation conditions - 0.3 0.3 0.5 0.5

4.15 Calculation and sizing results

The calculation results of the main parameters of the cooling model in different study locations are presented in Table 4.4. For sizing the pipes, a maximum velocity of 0.49 m/s is considered for the water inside the pipes. The fan coil efficiency is assumed as 98 %. The pumps are sized based on their loop flow rates. The values in the Table 4.4 are used to size pumps and pips in cooling loop, however; new models are defined and added to Polysun library for sizing the re-cooler and fan coil.

Table 4.4. Calculation results of system sizing for different study locations.

Component Unit Dubai Delhi Los Angeles Cape Town

Chiller capacity [kW] 13.45 12.65 10.40 9.85

Re-cooler capacity [kW] 58.27 54.80 31.70 29.55

Fan coil capacity [kW] 13.72 12.90 10.61 10.05

Flow rate - re-cooler loop [l/h] 7191 6763 3846 3647

Flow rate - fan coil loop [l/h] 2323 2185 1795 1702

Flow rate - generator loop [l/h] 3951 3716 1832 1737

Pipe - re-cooler loop [mm] 72 69 52 51

Pipe - fan coil loop [mm] 41 39 36 35

Pipe - generator loop [mm] 53 51 36 35

Pump - re-cooler loop [W] 78 78 50 50

Pump - fan coil loop [W] 50 50 50 50

Pump - generator loop [W] 50 50 50 50

(34)

4.16 Modeling of Solarus PV/T collector in Polysun

A new collector model was created based on thermal and electrical properties of PV/T manufactured by Solarus company. The technical characteristics of Solarus PV/T are presented in chapter two, table 2.4, Figures 2.13 and 2.14. The technical properties that used for the PV/T model are namely module gross and aperture area, PV efficiency, electrical power, output current and voltage at STC, open circuit voltage and short circuit current, zero-loss coefficient, heat-loss coefficient, second degree heat-loss coefficient, incidence angle modifier (IAM) and maximum collector temperature.

System Integration of PV/T Collectors in Solar Cooling Systems

Master Level Thesis

European Solar Engineering School No.198, August 2015