Small-scale biogas – A techno-economic comparison of Internal Combustion Engines, Micro Gas Turbines and Stirling Engines

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Small-scale biogas –

A techno-economic comparison of Internal

Combustion Engines, Micro Gas Turbines and

Stirling Engines

Emil Sund

TRITA-ITM-EX 2020:422

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ii Master of Science ThesisEGI 2020:422

Small-scale biogas –

A techno-economic comparison of Internal Combustion Engines, Micro Gas Turbines and Stirling Engines

Emil Sund

Approved

2020-08-20

Examiner

Björn Laumert

Supervisor

Rafael Guedez

Commissioner Contact person

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Abstract

Gas driven power generation is expected to increase in the near future due to significantly lesser environmental impact than oppose to conventional fuels like coal and oil. Today, Natural Gas (NG) is the dominant product on the market and biogas, which originates from degradable organic waste, has not reached its full market potential due to bottlenecks in utilization. Biogas often contain less methane and more trace elements than NG and is therefore seldom feasible to use in power generation. Especially at small biogas flows, the equipment needed for cleaning of the gas is often too expensive to consider. The biogas used in this study was assumed to contain 55 % methane with trace elements of hydrogen sulfide (H2S), moisture and Siloxanes. One way to surpass this cleaning step is to use externally fired machines, like Azelio’s 10 kWel GasBox which is a Stirling engine coupled with a gas preheater and combustion chamber.

A model was developed which analyzed both costs and operation parallel with the two most market mature options, Internal Combustion Engines (ICE) and Micro Gas Turbines (MGT), which indicated that the GasBox is competitive at small biogas flows translating to less than 100 kWel output due to this. A market analysis and screening with a Multi-Criteria Analysis (MCA) was performed to locate markets for which the demand for a product like the GasBox existed. The study showed that there is an estimated demand for around 53 000 units in the German agriculture, which is one of the world’s most innovative markets when it comes to both biogas and renewable energy in general. Further on, it was discussed what is needed from both understanding the market but also the technologies involved for succeeding on the market.

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Sammanfattning

Kraftproduktion från naturgas och biogas förväntas öka framöver på grund av dess avsevärt lägre miljöpåverkan än konventionella bränslen som kol och olja. För att biogas ska kunna övervägas som bränsle i traditionella kraftomvandlingsprocesser, krävs gasrening gasen generellt har lågt värmevärde och skadliga spårämnen. Biogas antogs vidare innehålla 55 % metan med spår av både svavelsulfid (H2S), fukt och Siloxaner. Vi små flöden är gasrening kostsamt, därför tittade denna studie på en extern Stirlingmotor kopplad med en gaskammare och förvärmare, Azelio’s 10 kWel GasBox. En modell utvecklades som visade på att GasBox är konkurrenskraftig vid små installationer under 100 kWel, jämför med de två mest använda lösningarna idag, interna förbränningsmotorer och mikroturbiner, mycket på grund av dess förmåga att drivas på lågvärdiga bränslen utan reningsbehov. Vidare undersöktes var och på vilka marknader GasBox kan tänkas konkurrera genom en Multi-kriterieanalys (MCA) som visade på att det uppskattningsvis finns ett behov av 53 000 enheter i Tysklands jordbruk, som är en mogen marknad gällande både biogas och förnybar energi. Vidare diskuterades vikten av förståelse för både produkt, behov och marknad, för att på sikt lyckas med energiprojekt i dessa skalor.

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Acknowledgements

I would like to thank my very gifted and driven academic supervisor Dr. Rafael Guedez for arranging this opportunity and giving me much needed support during this spring. Your insights and the way you untangle issues that arise within the field are inspirational. I would like to thank the people at Azelio for encouraging me during this thesis and being very including, giving me the inspiration I needed, especially Jan my supervisor who took his time to discuss the core issues surrounding the objectives of the report to help me forward.

As a concluding remark, no thanks or wishes can ever cover the gratitude I feel towards my family and my fiancée who has stood by me and are the reason I graduate from KTH in the manner that I do.

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Abbreviations

AD Anaerobic digestion

AF Availability factor CAPEX Capital Expenditure(s)

CHP Combined Heat and Power

DG Digester gas

FIT Feed-in tariff

GHG Green House Gases

GtP Gas-to-Power

GWP Global Warming Potential ICE Internal Combustion Engine IEA International Energy Agency

IRENA International Renewable Energy Agency IRR Internal Rate of Return

LCOE Levelized Cost of Electricity LCOH Levelized Cost of Heat

LFG Landfill Gas

LNG Liquified Natural Gas MCA Multi Criteria Analysis

MGT Micro Gas Turbine

MSW Municipal Solid Waste

NG Natural Gas

NPV Net Present Value

O&M Operation and Maintenance OPEX Operational Expenditure(s) PPA Power Purchase Agreement

RE Renewable Energy

RES Renewable Energy Systems SAM Serviceable Achievable Market SDG Sustainable Development Goal SOM Serviceable Obtainable Market TAM Total Addressable Market UAA Utilized agricultural area

UN United Nations

USD United States Dollar

WACC Weighted Average Cost of Capital

WtE Waste-to-Energy

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Tables

Table 1 - Suggested composition of natural gas (8). ... 5

Table 2 - Estimated costs for advanced landfill operation. ...10

Table 3 - Typical composition of LFG (29). ...12

Table 4 - Estimated costs for small-scale biogas (41). ...14

Table 5 - Biogas potential from livestock (42), (43). ...14

Table 6 - Typical CAPEX of small-scale biogas application. ...15

Table 7 - Typical composition of biogas from agricultural waste (48). ...15

Table 8 - CAPEX and OPEX for greenhouse operation. ...18

Table 9 - Summary of biogas upgrading technologies and their current status (62). ...20

Table 10 - Estimated CAPEX and OPEX for biogas purification technologies (48). ...21

Table 11 - GasBox specifications...28

Table 12 - Composition of the modelled biogas. ...34

Table 13 - Operational and technical requirements of the studied PCU's. ...35

Table 14 - Annual operating characteristics for the studied technologies. ...39

Table 15 - Thermal output (hot water) potential...40

Table 16 - Assumed OPEX for remaining technologies. ...41

Table 17 - Assumptions for LCOE calculations ...42

Table 18 - Estimated exhaust emissions. ...43

Table 19 - Results of the MCA screening. ...44

Table 20 - Top 10 scoring countries from MCA ...44

Table 21 - Baltic countries electricity overview. ...45

Table 22 - SAM for landfills in the Baltic countries. ...45

Table 23 - SAM Estimation in Baltic agriculture. (106), (107), (108) ...46

Table 24 - Summary on agriculture market potential in the Baltic states. ...47

Table 25 - German electricity overview. ...48

Table 26 - SAM estimation for Agriculture in Germany. ...49

Table 27 - Spread of agricultural holdings according to number of livestock units (LSU). ...49

Table 28 - SOM results in German agriculture...49

Table 29 - Financial input parameters for case study. ...50

Table 30 - Biogas composition used in case study. ...50

Table 31 - Conditions for chosen farm in Germany. ...51

Table 32 - Resulting KPI's for case study...52

Table 33 - Characteristics of the studied technologies. ...53

Figures

Figure 1 - Global investments in energy 2018. (2) ... 1

Figure 2 - Report focus areas... 3

Figure 3 - Waste management hierarchy (11). ... 6

Figure 4 - Estimation of MSW management in high-income countries (12). ... 7

Figure 5 - Estimation of MSW management in low-to mid-income countries (12). ... 7

Figure 6 - Unofficial material recycle at unsanitary landfill (16). ... 8

Figure 7 - Generic setup of sanitary landfill (19). ... 9

Figure 8 - High-and low-income MSW composition (12). ...10

Figure 9 - LFG generation potential (26). ...11

Figure 10 - LFG composition variation (28). ...11

Figure 11 - Energy use in Swedish agriculture (37). ...13

Figure 12 - Varying load curves on two different dairy farms in Sweden (39). ...14

Figure 13 - Plastic tunnel farming example ...16

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Figure 14 – Hydroponics, irrigation and nutrient flow is controlled. ...16

Figure 15 - Industrial greenhouse application...17

Figure 16 - Biogas upgrading technologies and their market share in Europe (62). ...19

Figure 17 - p-V and T-S diagram of the ideal Stirling cycle, modified from (64). ...22

Figure 18 - Piston placement in Stirling Engines (70). ...23

Figure 19 - Crank-drive ...23

Figure 20 - Operating principle of an alpha configuration Stirling engine with kinetic drive mechanism. ..24

Figure 21 - Process flow-chart of biogas-to-end-use. ...25

Figure 22 - Chosen systems to be modelled in the techno-economic model. ...25

Figure 23 - SI engine cross section (72). ...26

Figure 24 - PV-diagram for ideal Otto cycle (73). ...26

Figure 25 - MGT cross section (72). ...27

Figure 26 - PV-diagram for ideal Brayton cycle (88). ...27

Figure 27 - GasBox system configuration. ...29

Figure 28 - Methodology ...30

Figure 29 - Market size estimation ...31

Figure 30 - Global TPES 2017 (95). ...32

Figure 31 - Black-box modelling. ...33

Figure 32 - Comaparative model...33

Figure 33 - Schematic of biogas cooling process. ...35

Figure 34 - Psychrometrics chart for biogas cooling process...36

Figure 35 - Biogas filtering with Activated Carbon. ...37

Figure 36 - Conversion efficiency for GasBox. ...38

Figure 37 - Conversion efficiency for ICE. ...38

Figure 38 - Conversion efficiency for MGT ...39

Figure 39 - Resulting power curve fit for MGT. ...40

Figure 40 - Resulting power curve fit for ICE (<100 kW). ...41

Figure 41 - Resulting power curve fit for ICE (100< kW). ...41

Figure 42 - Highlighted map of MCA ...44

Figure 43 - Agricultural area in the Baltic countries. ...46

Figure 44 - UAA per holding, Baltic countries ...46

Figure 45 - Livestock in Baltic farms. (109), (110), (111). ...47

Figure 46 - Electricity mix in Germany 2018. ...48

Figure 47 – SAM and SOM for landfills in Germany. ...48

Figure 48 - Main areas for cattle management in Germany (116). ...50

Figure 49 - CAPEX split for MGT, ICE and GasBox...51

Figure 50 - Genset CAPEX for MGT, ICE and GasBox. ...51

Figure 51 - OPEX split for MGT, ICE and GasBox. ...52

Figure 52 - Genset OPEX for MGT, ICE and GasBox. ...52

Figure 53 - IRR vs. Size of installation...54

Figure 54 - IRR vs. biogas methane content. ...55

Figure 55 - IRR vs. Annual operating hours. ...55

Figure 56 - IRR vs. CAPEX. ...56

Figure 57 - IRR vs. OPEX. ...56

Figure 58 - IRR vs. WACC. ...57

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1 Introduction

The UN Sustainable Development Goals (SDG) are targets that together form a framework meant to simplify and enable the progress towards a more feasible approach to energy, natural resources and the development of society in general. The implementations made as a result, are now on their fifth year in place and every year they can be evaluated better than the previous, as they mature and progress. The UN SDG 13 reads “Take urgent action to combat climate change and its impacts” and today, still, the global progress is slow and climate change is occurring faster than previously anticipated with higher temperatures and sea levels as first symptoms. The way society has progressed, and increased consumption and production historically needs to shift faster towards more outside-the-box and locally feasible strategies to be able to curb the negative trend. In the matter of geopolitical issues and use of resources, developed countries today use one fifth of the world’s natural resources, leaving less developed countries struggling (1).

Every year, the International Energy Agency (IEA) publishes the World Energy Outlook. A substantial collection of information regarding the status of the world’s energy related activity, coupled with IEA’s own projected future scenarios. In the 2019 edition the gaps between the world’s ambition and its actions are being highlighted. The SDG’s previously mentioned, which includes SDG 7 and reads “Clean and reliable energy for all”, is at the risk of failing when in 2020 one billion people still do not have access to electricity at all. Same goes for the ambitious goals of global renewable energy fraction to increase, when investments in fossil-fuels and fossil-fuel activities still accounts for a large fraction.

Figure 1 - Global investments in energy 2018. (2)

The role of gas is expected to be of great importance, since it many times delivers more energy in its networks than the pure electricity networks due to high energy density. Natural gas (NG) is for now, and in the nearest future a quick solution to lower emissions from fossil-fuel based generation technologies since it can utilize, with small modifications, the same machinery. The NG market is an enormous industry today with pipelines, both domestic and international, running for vast distances. Many regions have large NG resources and the export market is a lucrative opportunity. Biogas is gas formed from more recent biological degradation and is mentioned, still, as an untapped resource. Biogas from waste is said to be able to cover

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2 20 % of the world’s gas demand today, with greatly reduced environmental impact than fossil-fuels but also NG. (3).

Gas-to-Power is usually in the form of combusting or burning the gas to generate electricity, through a boiler for steam generation, gas turbines or for smaller application gas engines. The use of gas instead of coal and oil is gaining attention due to its lesser environmental impact, and especially biogas has the potential to reduce greenhouse gas emissions (GHG) drastically when generating electricity, as previously mentioned.

Waste heat is nowadays a welcomed biproduct in these applications and is being incorporated and utilized to increase the overall efficiency of the installation, a Combined Heat and Power (CHP) solution. Though, the composition of biogas is of lesser quality than conventional fuels like gasoline or even NG and has great purification needs to be eligible for power generation through the previously mentioned technologies. Even NG needs extensive treatment to be eligible for sales through existing pipelines and as liquified natural gas (LNG).

Until now, the dominating low-quality gas application seen is the use in less developed countries, usually rural areas, where low quality gas obtained from anaerobic digestion (AD) is burned directly with a more robust burner to fuel cooking stoves and basic heating needs. This is a much healthier and environmentally friendlier option, oppose to the use of fuel wood. A way to further increase the utilization of raw biogas is to surpass the extensive cleaning and upgrading phases and to use externally fired power converting units (PCU), like the Stirling engine.

1.1 Azelio AB

Azelio is a Swedish cleantech company that, using a Stirling engine, are at the forefront of renewable energy solutions. In March 2020, the first installment of Azelio’s long-duration thermal energy storage had its inauguration in Ouarzazate, Morocco. One of the company’s already commercialized products is the GasBox, which is the Stirling engine coupled with a biogas burner and a gas preheater.

One of the benefits with the GasBox is that it’s able to run on the previously mentioned low-quality gases without the need of further treatment. The externally fired technology enables continuous combustion while the gas resource is available. This is particularly beneficial in for example remote and off-grid applications.

Grids today are experiencing capacity issues and more on-site generation will most likely be requested and the so called “demand response”, where the user side and the consumption adapts to the production instead of the other way around, can play a large role. This will not be feasible without the use of self-generated power. By concluding this, the motivation for the report is then to promote and bring forth less mature technology to see where such technologies can be a good compliment to already established ones.

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1.2 Objectives

The main objective is to investigate best business opportunities for waste-gas-, heat-driven combined heat and power and the technology used, like Azelio’s GasBox. The GasBox has historically not been able to scale up its sales and reach its full market potential. The objectives of this report are therefore to accurately assess the GasBox and put it in its correct context, identifying what its role in the gas market could be. To do this, a comprehensive study regarding the CHP market needs to be performed identifying key elements and market forces that will determine if the GasBox can compete with currently more implemented and mature solutions. Being a less proven technology, a techno-economical assessment needs to be performed in each of the identified potential business cases. To accomplish this, the following sub-objectives are:

• To perform a thorough review of current market conditions for waste-gas to CHP applications and technologies deployed.

• To develop a methodology to assess the market potential of Azelio’s GasBox including estimation of market size, identification of best business opportunities, customers and business models, and differentiation with competing technologies.

• To develop a techno-economic model to quantitatively assess the performance of Azelio’s GasBox under identified best business opportunities.

• To determine best go-to market opportunities based on results from case-studies and input from market study, and thereby provide recommendations for future technological developments and commercialization of the GasBox.

1.3 Scope & Limitations

There have been any investigations on small-scale biogas application and feasibility. This report will surpass the biogas generation phase up until raw biogas is available, since it has been shown many times that a high threshold is the high investment costs of the biomass-to biogas systems. Therefore, this study will focus on the raw biogas-to electricity phase., shown in Figure 2.

Figure 2 - Report focus areas.

The scope of the investigation is further limited since the GasBox is a 13 kWp unit capable of delivering steady operation at 10 kWel on biogas. Container solutions are possible, coupling several engines and reaching a higher power output. But this puts an upper limit on where the GasBox should be able to compete initially. The applications considered are therefore small-scale gas applications, where hazzle-free and simple installations are most probably preferable over complex ones. The report will focus on applications with capacity of 100 kWel. For knowledge and further understanding, larger installments have been studied but in a lesser extent.

Another limitation is the competing solutions considered. Seeing that the GasBox is a solution that is already on the market, only the mature technologies already in place for low-quality gas utilization will be considered, capable of delivering both electricity and heat in CHP operation. These are the internal combustion engine (ICE) and Micro-gas turbine (MGT).

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1.4 Thesis structure

To be able to structure the report and include all necessary theory, the following structure will be followed:

• Section 1: Underlying theory (chapter 2).

This section will handle all the theoretical concepts that the report will have to cover. The various products on the global gas market, it’s fundamentals and forces will be investigated and

summarized. Together with the thermodynamics, development and status of the Stirling engine, this will be the theoretical framework for the thesis.

• Section2: Methodology (chapter3).

For the reader, to be able to follow and understand the chosen methods and assess its strengths and weaknesses. All the way from the first approach literature study conducted, to the

conclusions drawn.

• Section 3: Market analysis (chapter 4).

The initial geographical scope for this report is wide and begins with a global screening in regard to gas business opportunities, to be able to identify most promising markets.

• Section 4: Techno-economic model and results (chapter 4-5)

When markets and actors have been sought out, a case-study to determine the strengths of the GasBox will be performed.

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2 Theoretical Framework and Background

Gas is a natural resource originating from several different applications. NG is a fossil energy source that is found deep inside the earth’s surface. Methane (CH4) is the largest component which is a combustible hydrocarbon. To deliver the end-product which is NG, the gas is often transported in pipes and cleaned of any trace elements. The gas is estimated to in the near future replace many conventional fuels due to its similarity to oil-based products like gasoline, but with lesser environmental impact (4).

Biogas on the other hand, is a methane rich gas formed from the anaerobic digestion (AD) of biomass.

Since the biogas is seen as an already present source of carbon dioxide (CO2) in the atmosphere, its environmental impact is close to zero. What inhibits its use is the quality of the gas which is, in terms of methane content, not as high as NG, making all related equipment more expensive and not as mature (5).

2.1 Gas market

The gas market in general is most often coupled with the oil market and has been analyzed according to the volatility of the oil price. Although, the US utilization of the vast reserves of shale gas during the last decade has somewhat decoupled the two markets. LNG plays a big role in the global trade, but the increasing number of trade sanctions together with a decrease in economic growth globally and lower oil prices has been forecasted to halt the growth seen during the last couple of years (6).

NG is today experiencing a large increase in demand, where both the US and China are the largest actors.

The two main factors for its rapid increase (4,6 % in 2018) is said to be due to the economic growth in Asia and China but also the urgency of switching coal-fired power generation to more environmentally friendly options. The largest sector for natural gas is the power sector and is expected to maintain its position during the coming years. One of the reasons the consumption increase can grow is the way of transporting the gas between regions of unequal natural resources. LNG trade has the market leading status when it comes to gas trade world-wide (7). Typical composition suggested by (8) can be seen in Table 1.

Table 1 - Suggested composition of natural gas (8).

Compound Molar

Fraction

Methane 0,75-0,99

Ethane 0,01-0,15

Propane 0,01-0,10

n-Butane 0,00-0,02

Isobutene 0,00-0,01

n-Pentane 0,00-0,01

Isopentane 0,00-0,01

Hexane 0,00-0,01

Heptane and longer 0,00-0,001 Non-hydrocarbons

Nitrogen 0,00-0,015

Carbon dioxide 0,00-0,30 Hydrogen Sulfide 0,00-0,30

Helium 0,00-0,05

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6 Biogas is reaching some level of market maturity in Europe with a total of almost 18 000 plants in operation in 2019. It is expected to play an important role in the Renewable Energy Directive during the coming decade with more incorporation of circular economy since the digestate of anaerobic digestion (AD) can be used as a bio-fertilizer which further increases profitability. The trend is however towards the production of Biomethane in contrast to on-site CHP (9).

2.2 Landfills

In and around cities and settlements everywhere, so called landfills have existed. Landfills are dumpsites at which many different waste fractions end up and degrade. It is considered to be the least favorable option for waste management, as seen in Figure 3, seeing that it causes emissions of all sorts, from odor to particle matter and can in many cases be harmful for the surrounding environment (10).

Figure 3 - Waste management hierarchy (11).

Methane is generated through the degradation of biological waste and is up to 30 times more potent as a greenhouse gas than carbon dioxide. At landfill sites, the collection of municipal solid waste (MSW) is usually the main activity in many places globally.

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7 (12) estimated that almost 50 % of the MSW generated in the world ends up in landfills, which can be seen in Figure 4 and Figure 5.

Figure 4 - Estimation of MSW management in high-income countries (12).

Figure 5 - Estimation of MSW management in low-to mid-income countries (12).

Later information indicates that these figures are an understatement. In under-developed countries, as much as 90 % (13) is estimated to end up in landfills and-or in so called dumps or dumpsites and even in the EU that figure could be as high as 60 % (14).

The main purpose of landfills will differ depending on where it is located. The typical activities at landfills are:

- Pile or burn waste to make room for more waste at the source of activity - Collect and sort materials for reuse and sales (both regulated and unregulated) - Collect and sort materials for recycle (both regulated and unregulated)

- Reduce environmental impact by centralizing waste disposal - Utilizing low-grade fuels for heat, power and high-quality fuels.

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8 In most cases, the operation of a landfill is a combination of the above-mentioned activities. In this report, landfills will be classified as one out of three possible types with one or several activities taking place:

i. Unsanitary landfills (Open dumps, uncontrolled and unregulated) ii. Sanitary landfills (Controlled and regulated)

iii. Advanced landfills (WtE or similar, controlled outputs)

The unsanitary waste disposal sites are of great concern both locally and globally. As previously mentioned, up to 90 % of undeveloped regions MSW ends up on these sites with hazards of many kinds as a result.

Environmental hazards due to toxins leaking out with water inside or rainwater (leachate), gas leakage with high global warming potential (GWP) etc. Informal waste management refer to the unregulated activities taking place at the sites where civilians collect materials such as plastics and metal for re-sale at local markets.

This activity is associated with great health hazards, due to the occurrence of pollutants, toxic waste and objects that not unlikely could cause physical harm like sharp edges and needles (15).

Figure 6 - Unofficial material recycle at unsanitary landfill (16).

These generally start out as dumpsites but end up as uncontrollable landfills since there is no official ownership to dictate restrictions. In the regions where these generally appear, the legislation is weak or non- existent, so costs are all external and hard to measure. These types are most-often located on government- owned land which enables a path-to-market for WtE technology through local, regional or state authorities.

For most developed regions in the world, there are legislations that make landfill operation controllable.

These are called sanitary landfills and what differs them from uncontrolled site the most is that in uncontrolled sites, there is no effort of concealing the content or protecting the local environment from exposure to the hazardous leakages that occur. This seriously affects the groundwater as one example, since the leachate will find its way downwards due to gravity. In sanitary landfills, this is not the case. There are some technical requirements to be met in order for the site to be considered as sanitary:

- Bottom liners

- Leachate collection and removal - Final covers

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9 In the matter of gas utilization, it must be recovered due to the otherwise imminent leakage or even explosion risk (17). Current practices of disposing of the highly flammable biogas is often by flaring the gas, since it is considered a cheap option and by burning the biogas you reduce the GWP of the emissions (18).

A common setup of a sanitary landfill is depicted in Figure 7.

Figure 7 - Generic setup of sanitary landfill (19).

In modern waste management facilities, the landfill is comprised of a small part of the business. The main activity and the one activity that is strived for is material recycling. But due to the waste’s shape, attributes or complexity the only option is sometimes to landfill it. Unfortunately, one obstacle can be that the financial reward is to low and therefore material is landfilled. The classification of the landfill is usually divided in categories. According to the Swedish “Miljöbalken” the categories are:

- Hazardous waste - Non-hazardous waste - Inert waste

Sanitary or advanced landfills as projects are normally divided in three steps: planning and construction of the landfill, operation and decommissioning. In both the operation and after the decommissioning phase, depending on the content of the landfill there can be significant waste gas production.

When a waste management station has reached this level of complexity, it is most often obliged to abide by a vast legislation. Testing of the waste, registering and substantial safety measures are required (20).

Operational characteristics will be site dependent since landfills occur all over the world. In all cases, in order to set up operation for a sanitary or advanced landfill large upfront (pre-construction) costs or capital expenditures (CAPEX) will be unavoidable. This includes applying for permits, performing land assessments, building infrastructure around the site etc. An estimate on costs associated with the development of what this report labels as an advanced landfill, is shown in Table 2.

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Table 2 - Estimated costs for advanced landfill operation.

Phase Explanation Unit Value Source

Upfront Site evaluation, land acquisition

and permits etc. [USD/acre] 0,75-1 000 000 (19)

Construction All costs associated with the actual landfill. Excavation, liners, piping

etc. [USD/acre] 150-450 000 (19)

GCCS* Gas Collection and Control system 24-35 000 (19)

O&M Annual operation & maintenance [USD/acre] (19)

Total of CAPEX % 50% (19)

GCCS - [USD/acre] 4 100 (19)

Closure/Post-

closure Costs associated with

decommissioning of the plant [USD/acre] 150-300 000 (19) Annual

maintenance Costs required to keep the site sanitary

and controlled. [USD/acre] 2-3000 (19)

*Gas collection and control system.

Electricity consumption in landfills have been estimated as a lifecycle electricity demand at around 0,5 kWh/ton MSW (21). This number is the handling of the waste until it reaches its end station, which is the landfill. After, the operative electricity expenses are pumping of leachate and gas, but also standard electricity needs like lights and building/facility requirements.

The feedstock of landfill gas (LFG) is globally grouped and labeled to form Municipal Solid Waste (MSW).

MSW refers to the refuse accumulated from residential and commercial activities. For this reason, the composition and therefore the characteristics of MSW are varying depending on the region it is being collected. Quantity is usually reported as kg/capita-day and can reach values above 2 kg/capita-day in some regions. The quantity and content are strongly correlated to income level, where an increase in organic waste is generally noticeable when income decreases. A generalization was made by (12) between high-and low- income countries and can be in Figure 8.

Figure 8 - High-and low-income MSW composition (12).

The quality of LFG is usually defined by two selected gases: Methane (CH4), Carbon-dioxide (CO2). Other components are Nitrogen, Oxygen and Sulfur. Gas collection and control system (GCCS) is in many countries mandatory for large-scale landfills which usually consists of both vertical and horizontal piping to accumulate the gas (22).

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11 The decomposable waste is the organic part, which therefore determine the quality and the methane potential. According to (23), everything from 150 to 500 m³/ton waste is a possible estimate for biogas production from landfills, depending on organic fraction. This figure was in the Nordic plants in the 90’s close to 4 m³ per ton and year which implies that LFG generation occurs for many years after the waste has been placed (24), even if collection efficiency has gone up. MSW per hectare of landfill area is estimated to be anywhere between 2-8000 tons/year (25). Where there is economic feasibility, energy is utilized from the LFG by various combustion processes, mainly through internal combustion engines (ICE) and boilers for heat. ICE’s requires a methane content of at least 35-40 %. During its lifetime, a landfill site produces not only less gas if decommissioned but also with varying and uncertain methane content (as low as 20 %) which is problematic for these types of energy conversion processes (26). This usually occurs after the site has been closed and no more MSW is being added. The site then faces two options which is either to shut down the energy conversion operation and flare the low-quality gas or to re-invest in gas upgrading techniques (27).

Figure 9 - LFG generation potential (26).

The degradation of the waste is considered to pass through different phases in the early years, before the composition stabilizes, as seen in Figure 10.

Figure 10 - LFG composition variation (28).

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12 Even though LFG composition stabilizes, the actual composition will vary due to incomplete and intermittent degradation which causes issues for non-continuous combustion devices. Typical composition of LFG suggested by (29) is shown in Table 3.

Table 3 - Typical composition of LFG (29).

Compound Volume [%]

Methane 45-60

Other hydrocarbons 0,01-0,6 Non-hydrocarbons

Carbon dioxide 40-60

Nitrogen 2-5

Oxygen 0,1-1

Ammonia 0,1-1

Sulfides 0-1

Hydrogen 0-0,2

Carbon monoxide 0-0,2

In addition to the components shown in the table, Siloxanes are another trace element which causes problems when combusted (30).

The collected LFG’s worth is a matter of the current market situation. Many sites already have contracts to sell its LFG to local energy stakeholders, decreasing sensitivity of market forces like natural gas and oil price.

As mentioned, LFG is only attractive when of high quality usually. This always raises the trade-off between the CAPEX for equipment for upgrading low- and mid-quality biogas versus the expected revenues from sales of high-quality. General CAPEX for LFG-to-energy has a wide range due to many factors but are normally in the range of 1400 to 5500 USD/kW and operational expenditures (OPEX) around 130 to 380 USD/kWh, depending on size of installation (31).

(32) reported that the weighted average for bioenergy projects globally was 2100 USD/kW, regardless of scale, complexity or size of installation.

External costs are costs not as easily quantifiable and often not included in financial projections. These are the costs for the social and environmental losses that occur as a result of a project, which can be loss of nature, biodiversity in the area etc. External costs are today being more and more internalized due to better methods of estimations, such as property market value before and after or the so called “willingness to pay”

measurement, which is the monetary value visitors can consider paying to visit the site. Loss of amenities for surrounding activities are costs usually associated with the landfill itself and could be an incentive to increase complexity of the site with the addition of waste-to-energy (WTE) equipment, for sites not already engaged in such activities. Costs have been estimated at around 20-50 USD/ton of CO2-eq that is released.

However, costs have up until recently been regarded as fair, but are now considered to be in the regions of 100-200 USD/ton which is at the levels on where carbon tax has been discussed to reside (33). These high number arise after considering that the pollution decreases economic growth world-wide.

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13

2.3 Agricultural and farm residues

A farm is defined as a single unit operating under a single management, where agriculture is the main source of activity. The farm can in many cases supply non-agricultural services as a supplement, but the main activities are:

- Growing crops (non-perennial or perennial) - Plant propagation

- Animal husbandry

In many applications, a mixture of these activities is run simultaneously (34).

The market value of farming globally is expected to be around 3,5 % of the world’s total GDP, with values reaching 60 % in less developed countries mainly in sub-Saharan Africa (35). A large waste fraction is obtained as a natural result of farming and animal husbandry. This waste fraction, if combined, works well for generating biogas. Farm and agricultural activities can be divided in two categories: Factory/industrial or traditional.

In many developed countries, like Sweden, farm sizes have grown during the last decades and can be characterized as industrial farms where the main activity is to increase sales and profits. This results in high livestock numbers and large fields of cultivation. This in turn leads to high waste accumulation and biogas potential. The operations are under regulatory control which means further incentives to utilize WtE technologies, due to energy efficiency programs and general guidelines regarding energy consumption (36).

Energy use in Agriculture can be diverse due to its many different purposes and sizes. In an attempt to map the energy use in Swedish agriculture, (37) conducted a statistical analysis where industries with more than 2 hectares were included and the results can be seen in Figure 11. The energy use includes heating and electricity but excludes vehicle usage.

Figure 11 - Energy use in Swedish agriculture (37).

As the figure shows, electricity consumption consists of barely 50 % in agricultural holdings due to the high usage of trucks and other large machinery. Continuing on Swedish agriculture and energy usage, studies have been made to estimate and to increase energy efficiency (38), (39), (40). A common estimation is that the electricity usage is around 700 to 1000 kWh and year per milk-cow, or livestock unit (LSU), for operation.

A milk-cow is often used as a measurement and discretization in agricultural statistics and is measured as one LSU which equals the on-site energy input required for one dairy cow.

14%

29%

2% 2%

9%

0%

44%

0%

Energy use in Agriculture

Oil

Wood and Woodchips Pellet

Grain Straw LPG NG Electricity Other

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14 Often there is also on-site energy demand excluding operation, like households or other off-duty consumption, which can be a complicating factor when studying energy consumption in agriculture.

Concerning the actual load profile, it is shown to differ according to the selected activity on the farm. Two examples are shown in Figure 12.

Figure 12 - Varying load curves on two different dairy farms in Sweden (39).

Biogas applications for these industries are well-established in developed countries but the biogas flows are usually very large due to large volumes of waste. Traditional farms tend to be smaller in application and can have less regulatory restraints, making it less likely to incorporate expensive technologies. On the other hand, a traditional farm may struggle more with economic profitability, where energy savings are maybe more acute.

(41) recently summarized both CAPEX and OPEX for small-scale biogas production in Turkey (<100 m³/day) which can be seen in Table 4.

Table 4 - Estimated costs for small-scale biogas (41).

Daily production [m³] LPG Equivalent value

[kg] CAPEX

[EUR] OPEX

[EUR/Year]

4 628 1 548 296

16 2511 4 324 1 676

32 5022 7 633 2 828

Estimates on biogas potential from the most common livestock today are shown in Table 5.

Table 5 - Biogas potential from livestock (42), (43).

Livestock Potential [m3/day]

Cow ~ 1-3

Pig ~ 0,1-0,3

Poultry ~ 0,01-0,03

As previously mentioned, farm operations can include many different activities. For biogas applications, a mix of cattle and crops is preferred due to more favorable composition of the degradable waste and as a result also the biogas. (44) estimated the costs for small-scale bioreactors with a larger geographical scope (<200 m3/day) which is the equivalence of a 50 m³ (100 m3/day) digester tank. Costs can be seen in Table 6.

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15

Table 6 - Typical CAPEX of small-scale biogas application.

Equipment and work Price

Site preparation 2 000,00 €

Bioreactor casing 8 500,00 €

Biogas storage system 8 500,00 €

Substrate supply and disposal systems 5 000,00 €

Heating and mixing system 2 500,00 €

Control unit and elements 1 500,00 €

Separator for the solid and liquid matter of the used substrate 1 700,00 €

Installation 3 000,00 €

Total 32 700,00 €

The authors also report a cost of 1700 EUR of biogas purification equipment and 6000 EUR for an unspecified cogeneration engine and OPEX of 2 350 EUR per year. Mistry and Pugh (45) reported plant costs of 125 000 € for a plant with 9 m³/h capacity and larger installations of 100 m³/h seems to decrease in costs per capacity. This gives an approximation of digester costs for various sizes. Depending on site location and level of complexity, a variance in the applications and costs is to be expected.

IEA reported in 2015 CAPEX values as high as 6000 €/kWel and as low as 1000 €/kWel for small-scale biogas facilities in Germany, France and Austria (46).

CHP applications are in use today, but the deployment scale-up is slow due to the complex systems with the need of high technical expertise. Resistance is met many times due to odor and dangers associated with the storage of combustible gas (47). Both CAPEX and OPEX can be considered high, to even get gas production going. Typical composition for biogas from agricultural waste can be seen in Table 7.

Table 7 - Typical composition of biogas from agricultural waste (48).

Compound Volume [%]

Methane 50-80

Non-hydrocarbons

Carbon dioxide 30-50

Hydrogen 0-2

Sulfides 0-1

Carbon monoxide 0-1

Nitrogen 0-1

Oxygen 0-1

Ammonia Traces

A fairly new commercial activity are greenhouses, which are meant to complement conventional agriculture.

This due to the potentially increased yields, as a result of greater control of important cultivation parameters like irradiation, temperature, air quality etc. The main activity is the cultivation of selected crops and usually an enclosed greenhouse area is dedicated to one or a few selected crops that thrive in the same environment.

There are many ways of constructing a greenhouse but main variables to control are the following:

- Ventilation - Heating/Cooling - Lighting/shading - Irrigation

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16 Control over these, together with high capital investments make greenhouse activities costly in terms of operational factors and energy use, but highly rewarding if output can be controlled and properly projected (49). Chemicals are mentioned as an important variable. The addition of CO2 in a greenhouse will enhance the greenhouse effect which is preferable in this case. Other chemicals like fertilizers and nutrient needs are of importance but will be omitted in this report. Segmentation by technology level is relevant in the case of energy expenditure and other costs associated with the application, seeing that it can differ a lot between the simplest cases and the more advanced.

Greenhouse in its simplest of forms is usually in the shape of tunnels or small domes. The application offers the most basic protection and shielding from the environment which is an absolute necessity in order to cultivate certain crops. Due to the low automation possibilities, it often results in suboptimal conditions and low yield (50).

Figure 13 - Plastic tunnel farming example

To be able to successfully cultivate crops, the basic parameters like irradiation, temperature and irrigation are of crucial importance. In a medium technology greenhouse, the addition of at least ventilating possibilities is deemed to be necessary and is often a first step to increase control over the operation and yield. The use of hydroponic systems could be a measure to in the process also control water usage (50).

Figure 14 – Hydroponics, irrigation and nutrient flow is controlled.

To be commercially viable on large scales, a high technology greenhouse is sought after. This means high levels of automatization and control of all affecting variables that could in some way affect the expected yield (50).

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17

Figure 15 - Industrial greenhouse application

Natural ventilation through shutters and vents often provides a cheap option to cover parts of the fresh air requirements. Due to the high degree of control and flexibility needed, forced flow through fans are most often necessary and costly in terms of energy expenditure. (51) estimated the ventilation need for different installations. The requirement per square meter and year varied between 4,6 and 6,1 kWh depending on flow rate and operational hours.

Some crops develop internal temperatures of 5-10 °C higher than the ambient in times of high solar irradiance, due to the longer light waves inability to escape the enclosed structure that is the greenhouse.

Ventilation, even of the forced kind is usually not enough to cover the cooling need during these occasions and many different approaches can be applied. Water evaporation-based technologies like psychrometrics, will increase the available cooling capacity but in terms add to the electricity consumption with more advanced ventilation systems. (52) reported increases in water consumption of up to 300 % per square meter, with the same magnitudes for air flow.

Heating requirements will differ depending on location, but the functionality of controlled heating have more benefits than just temperature control, seeing that it can eliminate other issues like disease infestations through humidity regulation (53). Heating requirement for three different European greenhouse sites in Italy, Spain and Turkey ranged from 35,6 to 56,1 kWh/m² and year. Applications differ, but plastic tubes with running water of about 60 °C are not uncommon, seeing that they can be placed directly in the soil and target the heat where it is needed, close to the crops.

The solar resource is of great importance when venturing into greenhouse activities. When speaking of mild climates and in the higher northern altitudes, the suns irradiation is not enough to sustain a healthy growth, which reaches is limit at the so called “compensation point” which is around 14-30 W/m² and 0,1 kWh/m² and day. To ensure healthy cultivation and high yields, a minimum of around 2 kWh/m²and day is usually applied (54). Wherever the irradiation is not sufficient, artificial lighting will be required. Still, system backup and redundancy when it comes to lighting will most probably be required in many applications.

Knowledge about location specific precipitation is of importance to estimate the need for additional water addition to the system. Rainwater will in many applications be stored on site and distributed by the use of pumps (54). (55) demonstrated that for a 300 m² greenhouse application, a water collection site of 610 m²

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18 and pumping power of 0,75 kW was satisfactory at 318 m³ of rainwater distributed for one year. Installation costs of the system was around 5800 EUR¹.

To further investigate the energy expenditure in greenhouse and agriculture activity, an evaluation of the greenhouse business potential in Ghana (56) and in Mississippi (57) where studied and are shown in Table 8.

Table 8 - CAPEX and OPEX for greenhouse operation².

Amiran Farmers Kit EnviroDome Mississippi

Effective area 120 m² 240 m2 214 m²

CAPEX 14 721 € 17 848 € 12 367 €

O&M 2 950 € 3 167 € 6 554 €

Water &

Electricity 162 € 162 € 1 860 €

Heating - - 1 264 €

Greenhouse residues are similar to the residues found in agriculture and farming activities. In biogas applications, the gas composition can be justifiably seen as equal as long as there is complementing manure production by cattle or other animal husbandry incorporation.

2.4 Biogas purification

Due to the mentioned composition of the types of biogas mentioned (DG, LFG) a process of purifying the biogas is necessary in many applications, especially if electricity is a sought-after end-product. Biogas purification consists of both cleaning and upgrading, which can at times be done in the same process to some extent. Being a cost-intensive operation, most upgrading equipment requires clean gas, free of Sulphur due to the otherwise fast degradation and the mechanical failures that follow.

Many biowaste-feedstocks are often rich in moisture. Already before the fermentation process, pre- treatment by heating is done in several steps to dry the feedstock. But the water content in the biogas can still be significant and the use of coolers to precipitate the water from the gas is often used to increase performance of a power converting unit. Biogas is often referred to as “saturated”, meaning that any compression of the gas will cause precipitation of water which can greatly interrupt operation (58).

The process referred to as cleaning is when toxic compounds, mainly H2S and Siloxanes, are targeted for removal. These can be removed in biological, physical and chemical processes. Hydrogen-sulfide (H2S) will together with the moisture in raw biogas, form hydro sulfuric acid (H2SO4) when combusted which is highly corrosive and toxic. This acid will severely damage components in internal combustion applications and most be cleaned to low concentration levels.

From various feedstocks, like industrial or health-and hygiene products, so called Siloxanes form when biomass is digested to biogas and can be present in the gas of anything between 1 and 400 mg/m³. Siloxanes are volatile methylated organosilicon compounds (VMS) which during combustion in engines or burners, form silicone dioxide which will eventually clog surfaces and decrease efficiency. Most common techniques to remove siloxanes from biogas is the use of activated carbon adsorption (AC), which can be of the regenerative and non-regenerative kind. Non-regenerative is more common for small-scale applications (30).

Adsorption relies on the adhesive properties of the selected material.

1 https://www.statista.com/statistics/412804/euro-to-canadian-dollar-average-annual-exchange-rate/; 1 EUR = 1,37 CAD 2013

2 https://www.statista.com/statistics/412794/euro-to-u-s-dollar-annual-average-exchange-rate/; 1 EUR = 1,11 USD 2016

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19 (59) described the three general cleaning approaches which will be summarized in this section.

During the fermentation process inside a landfill or a digester, air or oxygen can be added to force H2S to form elemental Sulphur. This is usually labeled as biological cleaning. Drawbacks of the process will be one;

explosion risk and two; that if too much air or oxygen is added, the fermentation process and so also the methanation process will be affected negatively. If air is used, the following upgrading processes will be more difficult due to the remaining Nitrogen that is hard to separate from CH4.

Aqueous sodium hydroxide scrubbing is a well-used physical cleaning technology of removing H2S and is built upon the rapid reaction between H2S and sodium hydroxide and it being more rapid than other compounds found in biogas (60). Another popular process is chemical absorption.

Chemical methods where iron ions are added to hinder the formation of H2S already in the digester space, is one of the simplest methods. This will create iron sulfide which will be drained from the digester along with the digestate. (61) reported that this type of cleaning can add around 0,2–0,3 €/kg of Sulphur removed, adding to in many cases already strained economic models.

When harmful compounds have been removed, the gas goes through the next phase to increase the CH4

content above the 50-60 %, upgrading. This is done mainly by removing the other large gas compound, namely CO2. (62) performed an extensive review about current status regarding different technologies for upgrading biogas, which will be summarized in the coming section.

In Figure 16, the current status of the technologies with the highest readiness level and their market share on the European market is shown. The European market has been assumed to be representative regarding market maturity, due to the previously mentioned high penetration of biogas plants and technology.

Figure 16 - Biogas upgrading technologies and their market share in Europe (62).

Since the report will focus on market related activities, the four technologies with the highest market volume will be summarized: Pressure-swing adsorption (PSA), Membrane distillation, Water scrubber and Chemical scrubber.

The most implemented technology for biogas upgrading today is what is known as “Water scrubbing”. Its elementary principle is Henry’s law, the different solubility for CH4 and CO2 (and remaining H2S) in water, which is many times greater for CH4. This causes the CH4 to dissolve into the passing water when a higher pressure is applied to the gas.

PSA is built upon adsorption which is the difference between the molecule’s adhesive abilities on solid materials, like zeolites or active carbon. An adsorption material is placed to obstruct the biogas and

0% 2% 5%

17%

21%

36%

21%

Biogas Upgrading in Europe

Cryogenic Other

Organic physical scrubber PSA

Membrane Water scrubber Chemical scrubber

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20 compounds like CO2 will be retained by the material, while the CH4 can flow through. Once the adsorbent material is saturated, the gas is released and recycled to extract more CH4.

The process of absorption is when a fluid is dissolved by a liquid (amine solutions or aqueous alkaline salts) or a solid, absorbent. A counter flow current of this solution will absorb CO2 in the process and can then be diverted and safely evaporated by a heat source, usually in the form of steam.

Membrane technology relies on pressure difference across and the permeability of selected membrane materials to separate the biogas compounds. The materials that enable the separation of CH4 and CO2 are cellulose acetate and polyimide and CO2, which has higher permeability than CH4, will pass through while the gas containing more CH4 will remain on the high-pressure side of the membrane. The technology is offered with one or more recycling steps since unfortunately, the gas on the low-pressure side will still contain high levels of CH4.

In Table 9 the summary of the respective technologies operating parameters and current status can be seen.

All technologies can be seen having the gas cleaning step as a pre-requisite, considering that H2S is as corrosive on the expensive upgrade materials as for internal combustion engine cylinders, for example.

Table 9 - Summary of biogas upgrading technologies and their current status (62).

Unit PSA Membrane

separation Water

Scrubber Chemical Absorption Consumption of

raw biogas [kWh/Nm³] 0.23–0.30 0,18-0,20 0.25–0.3 0,05-0,15 Consumption [kWh/Nm³] 0.29–1.00 0,14-0,26 0.3–0,9 0,05-0,25

Heat consumption [kWh/Nm³] - - - 0,5-0,75

Heat demand [°C] - - - 100-180

Cost - Medium High Medium High

CH4 losses [%] <4 <0,6 <2 <0,1

CH4 recovery [%] 96-98 96-98 96-98 96-99

Pre-purification - Yes Recommended Recommended Yes

H2S co-removal - Possible Possible Yes Contaminant

N2 and O2 co-

removal - Possible Partial No No

Operation

pressure [bar] 3-10 5-8 4-10 Atmospheric

Pressure at outlet [bar] 4-5 4-6 7-10 4-5

This study reports costs as a matter of available raw biogas output, and the feasibility rapidly increases when surpassing 2000 Nm³/h. CAPEX decreases from 4-6000 EUR/ Nm³/h to 1000, when reaching high capacities. OPEX is reported for small PSA units, ranging from 0,015-0,15 EUR/kWh of biogas.

Similar studies have published similar results where one is observed in Table 10.

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21

Table 10 - Estimated CAPEX and OPEX for biogas purification technologies (48).

Unit PSA Membrane

separation Water

Scrubber Chemical Absorption Electric energy

demand [kWh/m3biomethane] 0,46 0,25-0,43 0,46 0,27 Consumption [kWh/Nm³] 0.29–1.00 0,14-0,26 0.3–0,9 0,05-0,25 Heat demand - - - - High

Heat temperature [°C] - - - 120-160

CAPEX €/(m3biomethane/h)

100 m3/h 10 400 7 300-7 600 10 100 9 500

250 m3/h 5 400 4 700-4 900 5 500 5 000

500 m3/h 3 700 3 500-3 700 3 500 3 500

OPEX €/(m3biomethane/h)

100 m3/h 0,128 0,108-0,158 0,14 0,144

250 m3/h 0,101 7,7-11,6 0,103 0,12

500 m3/h 0,092 6,5-10,1 0,0901 0,112

CH4 recovery [%] 98 80-99,5 98 99,96

Pre-purification - Yes Yes Process

dependent Yes

Delivery pressure [barg] 3-10 5-8 4-10 Atmospheric

Many of these upgrading technologies require pre-cleaning of sulfur and other contaminants. The main hurdle in small-scale biogas treatment is the high capital costs of the equipment and some attempts to bring the costs down for less complex solutions have been made. Aravind et al. published a comprehensive review regarding gas cleaning for small-scale fuel cell applications, which in that case is similar to small-scale CHP.

High capital costs coupled with increased maintenance are the largest barriers (63).

2.5 Stirling Engine basics

The Stirling engine is an externally fired reciprocating heat engine, which enables it to run on a variety of fuels unlike the internal combustion engine (ICE) which requires high-quality fuels like gasoline or diesel.

The pistons are sealed-off from the ambient, and the working media inside is usually Helium or Hydrogen, but in the early configurations air was used.

The ideal Stirling cycle is the thermodynamic basis which the power converting unit is based upon. The closed cycle consists of four processes:

1 – 2: Isothermal expansion (Constant temperature) 2 – 3: Isochoric heat rejection (Constant volume) 3 – 4: Isothermal compression (Constant temperature) 4 – 1: Isochoric heat addition (Constant volume)

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22 Like the Carnot cycle, the processes are considered reversible meaning that it produces work when heat is added and provides heat/cooling when work is added (64). The ideal Stirling cycle is shown in Figure 17.

Figure 17 - p-V and T-S diagram of the ideal Stirling cycle, modified from (64).

The ideal Stirling cycles thermal efficiency is given by Equation 1, which is the same as for the ideal Carnot cycle.

𝜂_𝑇 =^𝑊̇^{𝑜𝑢𝑡,𝑛𝑒𝑡}

𝑄̇_𝑖𝑛 =^𝑄̇^𝑖𝑛^−|𝑄̇^𝑜𝑢𝑡^|

𝑄̇_𝑖𝑛 = 1 −^|𝑄̇^𝑜𝑢𝑡^|

𝑄̇_𝑖𝑛 = 1 −^𝑇^𝐶

𝑇_𝐻 (1)

In the early 1800’s Robert Stirling filed a patent for a so called ”thermal regenerator” with an alleged power output of 2 horsepower, pumping water at a remote quarry with low efficiency. The true invention of Stirling was the regenerator part, which enables a large share of the thermal energy, that would otherwise be lost to the environment, to be recycled to the system. During this time, the real competition was from the steam engines, which is another externally fired power unit. An effort was made many years later to replicate the regenerator, similar to a furnace in design and it was concluded that with the suggested design and design parameters, power output of 0.5 horsepower would be more feasible. This was later somewhat confirmed through found written sources, claiming to have been on-site experiencing the first regenerator (65). It had its advantages over the steam engine, one being the significantly lower explosion risk. Boilers for steam engines at this time was at its cradle in development and was known to have caused human casualties. The bottleneck of the Stirling engine was the available material at the time, cast iron, and its inability to work with high operating temperatures which was required for the higher power applications (66). Materials, heat- transfer, size and weight are issues that has arisen and continue to arise when transferring the ideal theoretical cycle to real-life applications, causing Stirling engines to move away from the high theoretical thermal efficiency. (67). In many cases it is also favorable to deliberately construct units with simpler working principles and mechanics than to attempt to realize the true thermal efficiency potential.

In the 1930’s, when the development of the ICE’s rapidly increased, some new designs sought to increase efficiency and find new markets (68). There among an attempt to enter the radio market, but once again it was eventually outmaneuvered by the new thermo-chemical battery technologies. Phillips, who was the company in charge of the Stirling project, acquired vast knowledge about the engine in the process and developed several patents, making the company valuable for the survival of the product. The Stirling engine was eventually commercially available in refrigerator applications, running backwards (66).

During the 20^th century, several efficiency increasing components were tried and incorporated in various success. The gap between its high theoretical efficiency, approaching the one of the Carnot, and what has

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23 been achieved is large due to complex phenomenon occurring within the field of compressible fluid mechanics, thermodynamics and heat transfer (69).

As a first level classification, how the pistons are placed to generate power are of importance. These categories are named alpha, beta and gamma types.

In alpha-type configuration the pistons are separated on either side of the regenerator, heater and cooler.

The pistons are therefore both transferring work. The configuration makes it easier to arrange several units in connection, since the expansion on the first unit will become the compression of the next and so on.

In beta-type, the compression and expansion are handled by one piston and one displacer. The displacer does not transfer work but instead displace the working fluid between the hot and cold.

Gamma-type engines are similar to beta-type with one piston and one displacer. In the gamma-type engines these two components are separated which gives more design freedom and to create certain type of engines, the displacer needs more space than the piston which is then enabled (70).

Figure 18 shows the three different pistons arrangements previously mentioned.

Figure 18 - Piston placement in Stirling Engines (70).

Crank-slider crankshafts are widely used in reciprocating engines and has great operating experience and reliability. The role of the crankshaft is to transfer the reciprocating motion of the pistons to rotational movement. Therefore, the power piston is connected mechanically to the crankshaft, as per the definition of a kinetic drive-type engine. This connection can be seen in Figure 19.

Figure 19 - Crank-drive

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24 Inside the closed system that is the Stirling engine, a fluid with specific properties is being compressed and expanded. The most common fluids used are Helium, and Hydrogen, due to them being light and their high heat-transfer capacities. Helium, a noble gas which is chemically stable, is sometimes more preferred due to Hydrogen being considered explosive but having more thermal potential in terms of power output and heat transfer (71). The use of Nitrogen is also considered, due to Helium and Hydrogen having very high diffusivity, making them harder to seal and more prone to leakages.

Referring to the thermodynamic processes described, the operating principle of a Stirling engine can be explained.

1 – 2: Heat is added, forcing the hot piston to move due to the working media expanding.

2 – 3: Most of the working media has been heated. Expansion continues in the cold cylinder, extracting more work.

3 – 4: Now, the working media is at maximum volume and starts moving into the cold cylinder where it cools down.

4 – 1: Nearly all the working media is now found in the cold cylinder and the cold piston keeps compressing it by momentum or other piston pairs on the same shaft.

Figure 20 - Operating principle of an alpha configuration Stirling engine with kinetic drive mechanism.

2.6 Competing solutions

After reviewing the literature on gas-to-power solutions, many proposed solutions have been found. An attempt to summarize the most common system setups is shown in Figure 21.

Small-scale biogas – A techno-economic comparison of Internal Combustion Engines, Micro Gas Turbines and Stirling Engines