Small Scale Polygeneration System for Hotels in Costa Rica

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Small Scale Polygeneration System

for Hotels in Costa Rica

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Master of Science Thesis EGI 2020:MJ232X

Small Scale Polygeneration System for Hotels in Costa Rica

David Vargas Masis

Approved 2020-09-04 Examiner Dr. Anders Malmquist Supervisor Moritz Wegener

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Acknowledgements

I would like to thank my family for the amazing support during all my studies, especially during the last two years in order to conclude my master studies. I would like to give a special thanks to my parents for always pushing me to be better and giving me all the help, I needed to succeed. Their guidance has made me the person I am today and without them nothing would have been possible, so this is for you.

To my brother, Leo, and my partner, Heidi, thanks for always being there for me during this time. Including moments in which I was giving up or down, you always helped me to move forward and push me to give my best. Being away from home was not easy, but with your help Heidi it was possible to enjoy and finish this master’s studies.

Thank you to my examiner Dr. Anders Malmquist for his expertise during this thesis study. A special thanks to my supervisor Moritz Wegener for his constant advice and contribution to this study. Thank you for your time and guidance.

In general, thanks to all the people that helped and supported me through this journey.

Pura vida,

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Abstract

In a world where energy consumption increases every year and the current system harms the environment, new technologies are necessary to cope with such intensive energy demands worldwide. In such an era, polygeneration systems are an innovative and sustainable solution for that problem. Polygeneration systems can simultaneously produce electricity, heating, cooling, hot water, potable water, and other services in smaller, more flexible, and more efficient ways. Small-scale polygeneration systems can also help with the decentralization of energy generation and with promoting the use of more renewable energy sources in the power generation sector.

In this study, a polygeneration system is proposed for an ecohotel in the Guanacaste region of Costa Rica. The ecohotel demand as well as the availability of local renewable energy resources were studied to size the components of the system correctly. The small-scale polygeneration system consists of a biomass gasifier and an internal combustion engine as prime mover, as well as PV panels, batteries, a biomass boiler, an absorption chiller, and a membrane distillation system. The outputs obtained from the system and to be used in the hotel are electricity, cooling, hot water, and potable water. The results obtained were positive from an economic and environmental perspective when compared to the national grid electricity system. The economic savings are of $410,268 per the system lifetime of 25 years, which represents a 27% margin difference. As for the emissions, 14.4 tons of CO2 are saved every year from going into the atmosphere

which represents a 38% yearly reduction.

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Sammanfattning

I en värld där energiförbrukningen ökar varje år och det nuvarande systemet skadar miljön, är ny teknik nödvändig för att klara sådana intensiva energibehov över hela världen. I en sådan tid är polygenerationssystem en innovativ och hållbar lösning för det problemet. Polygenerationssystem kan samtidigt producera el, värme, kylning, varmt vatten, dricksvatten och andra tjänster på mindre, mer flexibla och effektivare sätt. Småskaliga polygenerationssystem kan också hjälpa till att decentralisera energiproduktionen och främja användningen av mer förnybara energikällor inom kraftproduktionssektorn. I denna studie föreslås ett polygenerationssystem för ett ekohotel i Guanacaste-regionen i Costa Rica. Ekohotelbehovet och tillgängligheten för lokala förnybara energikällor studerades för att dimensionera systemkomponenterna korrekt. Det småskaliga polygenerationssystemet består av en biomassaförgasare och en förbränningsmotor, liksom PV-paneler, batterier, en biobränslepanna, en absorptionskylare och ett membrandestillationssystem. Energiflödena från systemet, vilka ska användas på hotellet är el, kylning, varmt vatten och dricksvatten. Resultaten är positiva ur ett ekonomiskt och miljömässigt perspektiv jämfört med det nationella elnätet. De ekonomiska besparingarna uppgår till 410 268 USD under en systemlivslängd på 25 år, vilket motsvarar en marginalskillnad på 27%. När det gäller utsläppen sparas 14,4 ton koldioxid varje år från att nå atmosfären, vilket motsvarar en minskning på 38% per år.

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Table of Figures

Figure 1. IEA Installed power generation capacity by source in the Stated Policies Scenario, 2000-2040, IEA, Paris. (IEA, 2019) ...10

Figure 2. Electricity generation share in Costa Rica 2019 (CENCE, 2019) ...11

Figure 3. Thesis plan schematic ...14

Figure 4. IEA, General Electricity Consumption. (IEA,2020) ...15

Figure 5. IEA, Electricity Consumption by sector. (IEA, 2020) ...15

Figure 6. IRENA, Renewable Energy Patents Evolution. (IRENA, 2020). ...16

Figure 7. IRENA, Electricity Generation of different Renewable Energy sources Worldwide. (IRENA, 2020). ...17

Figure 8. Energy flow of CCHP (last image) and normal system (first three images) (Wu & Wang, 2006) 19 Figure 9. Schematic CCHP system technologies (Moussawi et al. 2016) ...20

Figure 10. System schematic of a polygeneration system in the Mediterranean region in Spain (Rubio-Maya et al. 2011) ...21

Figure 11. Cost comparison of services supplied by the proposed PP's systems versus services in the market (Villaroel-Schneider et al. 2020) ...22

Figure 12. Comparison of NPC, capital investment and CO2 emissions for all four cases (Wegener et al. 2019). ...23

Figure 13. Carbon cycle in a hydropower reservoir (IHA, 2020) ...24

Figure 14.GHG emissions intensity (g CO2 -eq/kWh) by climate region (IHA, 2018) ...25

Figure 15. Price of electricity for commercial and business in Costa Rica (ICE, 2020) ...25

Figure 16. Comparison of commercial and industrial electricity prices in Latin America (Osinergmin, 2018) ...26

Figure 17. Comprehensive framework of HOMER optimization procedure (Bahramra et al. 2016)...27

Figure 18. Location of hotel for the case study (Google Maps, 2020) ...28

Figure 19. Wind speed in Costa Rica (Global Wind Atlas, 2020) ...29

Figure 20. Solar irradiance in Costa Rica (European Commission PVGIS, 2020) ...30

Figure 21. Monthly electrical demand for the hotel in 2019 ...32

Figure 22. Electrical hourly data for the full year in 2019 ...33

Figure 23. Demand profile for March 2019 ...34

Figure 24. Demand profile for October 2019 ...34

Figure 25. Energy demand of the hotel in March ...35

Figure 26. Energy demand of the hotel in October...35

Figure 27. Demand data for the hotel in the case study ...36

Figure 28. System schematic ...36

Figure 29. Economic results ...39

Figure 30. Yearly CO2 emissions for both cases ...40

Figure 31. Cost of Electricity for both cases...40

Figure 32. Detailed cash flow for proposed system ...41

Figure 33. Economic comparison of all configurations ...42

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Figure 35. Cost of Electricity for all systems ...44

Figure 36. Cost per month comparison with the COVID-19 pandemic situation ...44

Figure 37. Sustainable Development Goals (UN, 2020) ...46

Figure 38. Location selection for case study in HOMER pro ...59

Figure 39. System Schematic of Proposed System in HOMER Pro ...59

Figure 40. NPC calculated by HOMER Pro for Proposed System ...59

Figure 41. System Schematic of Grid Case in HOMER Pro ...60

Figure 42. NPC calculated by HOMER Pro for Grid Case ...60

Figure 43. System Schematic of Grid Case with Covid-19 prices in HOMER Pro ...60

Figure 44. NPC calculated by HOMER Pro for Grid Case with Covid-19 prices ...60

Figure 45. System Schematic of Only Biomass Case in HOMER Pro ...60

Figure 46. NPC calculated by HOMER Pro for Only Biomass Case ...61

Figure 47. System Schematic of Only PV Case in HOMER Pro ...61

Figure 48. NPC calculated by HOMER Pro for Only PV Case ...61

Figure 49. System Schematic of System with Wind Turbines Case in HOMER Pro ...61

Figure 50. NPC calculated by HOMER Pro for System with Wind Turbines Case ...62

Table of Tables

Table 1. Summary of benefits and drawbacks of DES (Alanne & Saari, 2006). ...18

Table 2. Annual Potential for biomass gasification in Chorotega region ...31

Table 3. Price of bagasse per hectare ...31

Table 4. Price of bagasse per ton ...32

Table 5. Water demand estimation for the hotel ...35

Table 6. Summary of system component costs and characteristics ...38

Table 7. Proposed system component sizes...39

Table 8. Electricity generation ...41

Table 9. Different configurations component sizes ...42

Table 10. Results of proposed system for Guanacaste region ...48

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Abbreviations

AB: Auxiliary Boiler A/C: Air Conditioning AC: Alternate Current AbsC: Absorption Chiller

BG: Biomass Gasification System COE: Cost of Energy

CHP: Combine Heat and Power

CCHP: Combine Cooling, Heating and Power CCHPW: Combine Cooling, Heat, Power and Water

CRF: Capital Recovery Factor

CST: Certification for Sustainable Tourism Program

DC: Direct Current

DES: Distributed Energy System DG: Distributed Generation DH: District Heating

DEH: Domestic Electrical Heater

DEWH: Domestic Electrical Water Heater ELC: Electric Load Control

FEL: Follow Electric Load FTL: Follow Thermal Load

GHG: Greenhouse Gas Emissions

HOMER Hybrid Optimization Model for Multiple Energy Resources

HX: Heat Exchanger

IFMT: Internally Fired Microturbine ICE: Internal Combustion Engine LCA: Life Cycle Assessment LCOE: Levelized Cost of Energy LHV: Low Heating Value MD: Membrane Distillation NPC: Net Present Cost

O&M: Operation and Maintenance PES: Primary Energy Sources PP: Polygeneration Plants PV: Photovoltaics (solar) RE: Renewable Energy

RES: Renewable Energy Sources RO: Reverse Osmosis

SD: Sustainable Development

SDG: Sustainable Development Goals SG: Smart Grid

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1 Introduction

Our generation has a challenge in the coming years: to slow down, or better yet, stop the deterioration of our planet. In 2015, the Paris Agreement was signed by many global leaders promising to keep Earth’s temperature increase well below 2ºC to slow down the effects of climate change; however not enough changes have been made in order to make that happen. In the 2019 World Energy Outlook report by the International Energy Agency, current scenarios are compared to sustainable development scenarios and it shows that a lot of changes need to take place in order to keep the temperature rise under control (IEA, 2019).

Figure 1. IEA Installed power generation capacity by source in the Stated Policies Scenario, 2000-2040, IEA, Paris. (IEA, 2019)

One of the most important tasks is to shift from fossil fuels to renewable energy systems. As seen in Figure 1, the projections are promising for renewable energy sources in the Stated Policies Scenario. This scenario provides a detailed sense of the direction in which existing policy frameworks and today’s policy ambitions would take the energy sector until 2040. It used to be called the New Policies Scenario, but the name was changed in 2019 to iterate that it considers only specific policy initiatives that have already been announced (IEA, 2019). Even though the share of renewable energies such as solar PV, hydro and wind would increase very rapidly under this Stated Polices Scenario; coal and gas would still be a big part of the energy generation in the coming years, thus causing significant amounts of greenhouse gas (GHG) emissions, accelerating the deterioration of our planet. For the necessary energy transition to happen more rapidly, governments need to make law enforcements accelerate the shift towards energy generation by renewables and focus on energy efficiency and the decentralization of energy (Solomon, 2011).

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decentralize this energy production by using renewable energy systems and smart grids as the new era of energy infrastructure. Decentralized systems offer numerous advantages over centralized ones: reduced costs for transmission systems, energy efficiency gains, lower grid loss, higher resilience due to more reliance on distributed generation (DG) from small scale providers and larger share of renewables in the local energy mix (Sims et.al, 2007).

For these reasons, this thesis focuses on the decentralization of energy using small scale polygeneration system. The polygeneration concept combines different energy sources as input and produces several energy services as output, within one enclosed system, in a lucrative, sustainable, and efficient way. Specifically, this study concentrates on the hotel sector in Costa Rica and how it can be powered by renewable sources. Costa Rica is a small country in Central America that has ample natural resources. As a result, the country produces most of its energy from renewable sources, however a large portion of that share comes from hydropower, as the mix is not very diversified.

Figure 2. Electricity generation share in Costa Rica 2019 (CENCE, 2019)

As seen in Figure 2, hydropower represents 69.2% of the total energy production share, wind energy accounts for 15.8% and the remaining energy sources such as biomass, solar and geothermal only represent 15% of the total share (CENCE, 2019). This graph indicates that there are other resources available in the country which are not being exploited to their full potential. Those resources can be utilized to produce electricity in small scale polygeneration systems as decentralized systems for hotels in Costa Rica. The following sections will introduce polygeneration systems, elaborate on the impact of ecotourism on Costa Rica’s economy, and outline the goals and purpose of this study.

1.1 Polygeneration Systems

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In general, typical outputs of polygeneration systems are energy services such as: electricity, heat, cooling, and purified water. The inputs of polygeneration systems vary depending on the different resources found in the regions and where they will be developed. However, the objective is to use renewable sources such as solar, wind, biomass, hydropower and geothermal to produce a system that generates low emissions. To produce heat, a combustion system is needed, which can be fueled by biomass, for example. Normally, electricity is the primary service and heat is a secondary output. This excess heat can be utilized for additional energy services such as heating, water purification, or even for cooling systems using an absorption chiller (AbsC). Also, it is important to emphasize that to accommodate the electricity demand, which varies throughout the course of a day, a combination of energy systems and storage systems is essential.

1.2 Ecotourism in Costa Rica

Ecotourism is defined as responsible travel to natural areas that conserves the environment, sustains the well-being of the local people, and involves interpretation and education (Hearne, 2001). Costa Rica is a developing country, in which 25% of the total territory is protected as natural reserves and parks, and it is estimated that 5% of the world’s total biodiversity can be found within its borders (ICT, 2020). Because of Costa Rica’s abundant nature and biodiversity, many tourists visit the country to explore nature and be close to its flora and fauna, which is one aspect of ecotourism. The tourist that visit the country contribute to the development of the communities in areas around nature reserves and the entire economy. Tourism represents 8.2% of the GDP of Costa Rica and this translates to 8.8% of overall employment in the country. This represents an economic income of $3,832,000 in 2018 (ICT, 2019).

As one can see, tourism accounts for a large part of the economy in Costa Rica, and thus the focus of this thesis is to create a small scale polygeneration systems for the hotel sector in order to help the tourism sector advance in a more sustainable way by using decentralized renewable systems. Many hotels in Costa Rica use sustainable practices to reduce their impact on nature on the surrounding areas. Another way to make hotels more sustainable is to produce their own electricity from renewable sources. Depending on the hotel’s location, different sources could be used, such as: wind, solar, biomass, hydropower, geothermal and hydrogen. In doing so, the energy production would have lower GHG emissions, and the impact on nature would be less, as the systems would be small, thus decreasing the disturbance to the flora and fauna in the region.

1.3 Goals and Objectives

In this thesis study, the aim is to analyze the resources that are present in the western coast of Costa Rica and understand how these resources can be combined into a polygeneration system to obtain electricity, cooling, potable water, and heated water. The polygeneration system will be developed for a sustainable hotel in Costa Rica, as polygeneration systems can be an opportunity for hotels to produce their own energy in a renewable way which produces low emissions. This could in turn attract more tourists and help to protect flora and fauna in these regions, which is important for the growth and development of the ecotourism sector of the country.

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2 Methodology

In this section, the research approach adopted in this study is explained and analyzed. The research method used in this study is a quantitative approach that combines descriptive and experimental research. The study is based on a thorough literature review to explain the manner in which electricity is being produced nowadays and how it is affecting the environment. Based on the literature review, the problem is identified and a solution in the form of a decentralized system is proposed. Finally, the new system is compared to the current centralized system to draw conclusions about the feasibility and benefits of the proposed system. In order to perform this quantitative research, some processes need to be considered to achieve the objectives as proposed before. The following sections explain those processes which are system boundaries, limitations of the study, literature review, the research approach, and the results and discussions.

2.1 System Boundaries and Limitations

In this thesis, a case study is analyzed, so the system boundaries are constrained to the energy input and output of the system. The boundary starts at the analysis of the demand data to cover the required energy load and finishes at the energy produced in the system. The data utilized in this study is real data that was provided by an energy consumer – in this case a hotel in Costa Rica – for a full calendar year. That data is then used to design a system that meets the energy demand through the system’s output. would be to add a thermally driven membrane distillation unit that can be used to purify water. Is important to also mention that in this thesis, the focus is on a small-scale system, the systems are classified based on the capacities of the prime mover. Small-scale systems have been defined as systems with a capacity of 20kW-1MW (Maraver et al. 2013). Other boundaries of the study are that only renewable energy resources are considered, the system is developed specifically for a region in Costa Rica, only small-scale system is designed, and it is specifically designed for the hotel sector.

In the power system analyzed, it is necessary to consider some limitations in the process. The polygeneration system is designed taking into consideration three main factors: the users, the electricity demand, and the thermal demand. Due to the context of this study – a hotel – it should be noted that the users of energy are not constant or consistent over the entire time period during which the data was gathered. Users fluctuated constantly throughout the time period represented, and their usage behaviors undoubtedly varied, which can affect the demand data. This is different, for example, than if this study took place in an apartment building, where the users would be more constant for a substantial period of time. This factor affects the electricity and thermal demand data, in turn affecting the entire system, which needs to adapt to that dynamic behavior. Another limitation of the data collection for the case study is that in Costa Rica, there is no distinction between the thermal and electrical loads on the energy bill. The energy bill presents one number for total energy usage, without distinction if that energy was used for electrical, heating, or cooling purposes. This means that some assumptions were necessary to obtain data for the cooling, water purification, and hot water demand in the hotel. Finally, another limitation in this study relates to the components of the system. The software used in the study, HOMER Pro, has data about each component that is used in the calculation. Thus, some criteria cannot be changed easily.

2.2 Literature Review

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how small scale polygeneration systems can make a difference in that country. After explaining the situation from a general perspective, a more detailed review is done about each component of the system proposed. A study is presented about the characteristics of small scale polygeneration systems and how they can be integrated into the energy system in a sustainable and feasible way. Finally, a review on how the system will be able to function with regards to the operational strategy and the control of the system is analyzed.

2.3 Research Approach

The first and very crucial part of the research approach for this study is the data collection, which is the base to be able to design and scale the polygeneration system accurately. In order to collect the data, contact was made with a hotel representative in Costa Rica, thus obtaining real demand data for a full calendar year was possible. As mentioned before, since the data obtained only included electrical and was not divided into the thermal usage by the hotel, some research was performed in order to obtain a percentage of the energy that is used by hotels for cooling and water heating. This allowed for an estimate of the thermal load in the case study to be made. After reliable data is gathered, the design process can be done to properly configure the system with the different components. This is a crucial step in the case study, since the system configuration is responsible for the output results and thus, the final outcome of the project. To finalize the proper system configuration, some research regarding the best control strategy for the proposed system need to be performed, to ensure the system covers the electric and thermal demand. Finally, a sustainability analysis is performed to compare the system proposed in the case study with the centralized national grid system currently used by the hotel.

2.4 Results and Discussion

The results of this case study are achieved when the system can fulfill the thermal and electrical demand of the hotel. To obtain substantial results, the demand data for full calendar year was collected, and the system will be designed and tested using HOMER Pro Software in order to have techno-economic results depending on the different technologies used, and to be able to compare configurations. For a better explanation of the system variations, a discussion regarding daily energy behaviors will be conducted. By doing so, the results can give better insight on how much energy is used for electrical purposes and thermal purposes depending on the time of the day and systems used. Also, the results will be compared considering the different seasons of the year as to ensure comparable data. An expected outcome of this study is that a sustainable and reliable model that can be replicated by other hotels will be developed. The proposed system will be compared to the currently used systems from a sustainability, environmental, financial, and social perspective. Finally, some recommendations can be made regarding the future implementation of a system such as the one proposed for hotels in Costa Rica and how that can be beneficial for both the country and the individual users.

Figure 3. Thesis plan schematic

2.5 Sustainability

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to the objective of establishing more powerful economies. Developed countries decided to excessively burn fossil fuels to produce cheap energy in order to power their economies. Historically, economic development has been strongly correlated with increasing energy use and the increase of greenhouse gas (GHG) emissions (Sathaye et al. 2011). However, in the last decade, many countries have broken that correlation which means that counties can continue to develop their economies without damaging the planet (EEA, 2012). Also, in some developing countries, the demand for energy is growing rapidly in order to catch up with other economies, and it is important that this energy production is done in a more sustainable manner through renewable energies and more efficient systems.

Figure 4. IEA, General Electricity Consumption. (IEA,2020)

As seen in Figure 4, the electricity consumption worldwide has been increasing steadily, and in the last 20 years the demand has doubled. The energy demand is expected to continue growing in the coming years, so some measures will need to take place to allow for less polluting growth. In order to achieve a sustainable energy system, a transition away from energy sources with high greenhouse emissions is required. Several energy resources are available to meet our needs, and technology pathways for making this transition exist (Benson & Orr, 2008).

Figure 5. IEA, Electricity Consumption by sector. (IEA, 2020)

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both residential and commercial, the building sector accounts for about 35% of total energy consumption in the world (Wang, et. al, 2015). In order to respond to the rapid growth in the electricity consumption, more sustainable electricity production needs to take the lead in transitioning to more efficient systems. Buildings have great potential for energy saving and emission reduction, as it has been predicted that 25% of the CO2 emission reduction in 2030 will come from the building sector (Brandoni & Renzi, 2015). To

achieve that reduction in GHG emissions, more energy efficient technologies need to be used, but there is also an opportunity to increase energy generation from renewable sources and decentralize the energy production. In Figure 6, an exponential increase in the number of patents for new technologies in the energy sector is shown. With a heavy emphasis on solar PV and wind energy technologies leading the way in the coming years towards more sustainable electricity generation.

Figure 6. IRENA, Renewable Energy Patents Evolution. (IRENA, 2020).

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3 State of the Art

3.1 Decentralization of Energy

Decentralized energy generation means that energy conversion units are situated close to energy consumers, and thus, large power plants are substituted for smaller ones. A distributed energy system (DES) is an efficient, reliable, and environmentally friendly alternative to the traditional energy system (Alanne, 2006). An important detail about decentralized energy generation is that the transmission losses are very small because energy is generated closer to the consumers, so there is no large transmission infrastructure and the losses are reduced. The decentralization of energy production is an opportunity for renewable energy sources to be implemented in some areas were electricity and thermal energy are needed by local users (Wu & Wang, 2006). Renewable energies are becoming cost competitive very quickly and have the advantage that they can be deployed in some areas where building a large energy facility is not feasible. As Aichmayer et al. (2014) state, worldwide growth in electrical demand is mainly due to new customers in rural areas, which often lack access to a conventional electricity network. As Figure 7 illustrates, renewable energy technologies have been growing at a rapid pace in recent years and have been becoming more cost effective in combination with the DES concept.

Figure 7. IRENA, Electricity Generation of different Renewable Energy sources Worldwide. (IRENA, 2020).

In Figure 7, it can be observed that the electricity capacity from renewable energy sources has doubled in the last decade, with hydropower still being the main source of renewable electricity globally. As expected, the two technologies with the largest impact on the total generation are wind energy and solar PV, but it is also important to mention that biomass still accounts for a sizeable portion of this renewable production. Another advantage that a DES can offer is that it can be adapted depending on the resources present in a specific location. The combination of Renewable Energy Systems (RES) such as solar PV, wind, and biomass could achieve a total fulfillment of the demand for electricity, cooling, and heating energy. The energy sector is evolving rapidly and undergoing a disruptive transformation fueled by decentralized renewable electricity generation. Since 2012, renewable generating capacity has exceeded that of

non-Hydropower (mix plants)

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renewables by a widening margin. Similarly, renewable energies have had a positive impact on the provision of electricity access. Out of the people who have gained access since 2000, 27% have been reached through on-grid renewables, and 3% through minigrid and off-grid renewables (IEA, 2017). That shift into more RES and decentralization can be challenging for the systematic transformation of the energy sector. Maintaining business as usual in the energy sector will not allow for the mobilization of renewable energy to its full potential. Comprehensive regulatory, legal, and financial frameworks will need to enable a decentralized and proactive citizen-oriented organization of the energy sector with high shares of renewable energy (UN, 2018). The RE technologies have evolved so successfully in the last decade that the technological advantages are clear. One major challenge now lies in the regulatory and legal sector. A significant task for governments around the world is to set clear regulations for the implementation of RES and DES at full capacity. The World Bank estimates that only 40% of countries have a grid code that includes variability of renewable energy (RE), 36% of countries have transmission pricing rules for RE, 14% of countries have plant forecast rules for RE generation, and only 8% of countries have power exchange rules for balancing areas, which is an important consideration for feasibility of grid-connected RE projects (World Bank, 2017). Those numbers need to increase rapidly so that the RES and DES can be deployed quickly and easily in the years to come. Renewable energy technologies in DES offer the opportunity to contribute to a number of important sustainable development goals: (1) social and economic development; (2) energy access; (3) energy security; (4) climate change mitigation and the reduction of environmental and health impacts (Sathaye et al. 2011).

Table 1. Summary of benefits and drawbacks of DES (Alanne & Saari, 2006).

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3.2 Explanation of Polygeneration Systems, CHP and CCHP

As mentioned before, a polygeneration system has been defined as a system with more than three energy services as outputs. These could be electricity, heating, cooling, and water purification, for example. A well-designed polygeneration system maximizes the utilization of the energy consumed (Buonomano et al. 2014). However, polygeneration is still a very new term in the energy sector, while combined heat and power (CHP) and combined cooling, heat, and power (CCHP) are terms that have a longer history in the energy sector. These concepts became of strong interest in the energy sector because the efficiencies can increase when more than one product is obtained from the same system. As Moussawi et al. (2016) mentioned, electricity, heating and cooling are the three main components constituting the tripod of energy consumption in residential, commercial, and public buildings around the world. So, it makes sense to obtain all those products from the same system, which is why the CHP and CCHP systems began to be implemented around the world. These types of systems started to be deployed and gained more interest at the beginning of this millennium. The IEA reported that in 2007, CHP systems produced approximately 9% of global power generation (Cho et al. 2014). CHP systems have also been estimated to be responsible for 15% reduction of greenhouse gas emissions between 1990-2005 (IEA, 2008). Those findings paved the way for the introduction of CCHP and polygeneration systems in the coming years. Figure 9 shows how the efficiencies of these systems increased tremendously, comparing a CCHP to a “normal system” to produce heating, cooling, and electricity.

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obtain the same output only 100 units of fuel are needed, and the energy losses are reduced to only 19 units. This makes the CCHP far more efficient compared to systems that work independently, which is one of the main goals of polygeneration systems.

It is important to mention that polygeneration systems can be powered by both fossil fuels and renewable energies, in this thesis the focus is on the implementation of RES. As Rong & Su (2017) emphasize, polygeneration offers a potential to fulfill the ambitious target of zero energy buildings (ZEB), because the interdependence of different energy products can be utilized as an advantage and provide flexibility. Thus, they can accommodate more renewable energy sources in the system. The integration of renewables into polygeneration systems is very useful in terms of efficiencies as well, as Yousefi et al. (2017) concluded. Using an optimal CCHP system integrated with PV panels leads to a 40.8% reduction of energy costs, 38.7% reduction of the primary energy consumption and 72% reduction in emissions. The use of renewable technologies is of great interest for polygeneration systems, but in order to produce electricity, a prime mover is needed. That is where the use of biomass as a fuel becomes beneficial for these systems, as it can be used in a variety of prime movers: Stirling engines reciprocating engines, steam turbines, fuel cells and gas turbines (Maraver et al. 2013). In addition, biomass combustion is a carbon-free process as the resulting CO2 would be previously captured by the plants being combusted, and will be recaptured by future plants

(IEA, 2007).

Figure 9. Schematic CCHP system technologies (Moussawi et al. 2016)

Figure 9 presents a schematic of a CCHP system technology. In this example, the electricity production is presented only by prime movers and not RES. However, the use of RES can be a main part of the polygeneration systems. Hybridizing renewable energy technologies with CCHP systems based on conventional energy sources has come into the spotlight, because the two kinds of technologies can be used in a complementary way (Yang & Zhai, 2019).

As Rong & Su (2017) mentioned, current challenges of small-scale polygeneration systems are:

1. First, it is not easy for a small-scale polygeneration plant to operate as efficiently as a large-scale one when considering only the electric output.

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3. Finally, small-scale systems have big upfront investment while the payback time is not short. In spite of the challenges of small-scale polygeneration systems, the benefits could outweigh the drawbacks. It is true that they are less efficient than large scale systems when only the electrical output is measured, but that is the reason why is a polygeneration plant has multiple outputs, so the efficiency can be increased to higher levels. To overcome the significant upfront investment and payback time, governments need to create new policies and regulations to make these systems more cost competitive as a way to attract more investors to change to these type of generation systems, which have reduced GHG emissions.

3.3 Studies of Small Scale Polygeneration Systems

As previously mentioned, polygeneration systems are a recent concept in the energy sector. Many studies have been done to determine the benefits that polygeneration systems have in comparison to CHP, CCHP and traditional ways of serving electricity, heating, and cooling. For example, a study performed by Baghernejad et al. (2016) in a controlled environment with an ambient temperature of 21° Celsius and a solar irradiation of 800 W/m2 showed that a biomass-solar polygeneration system has an electrical efficiency of 57.6%, but when the overall efficiency of the system is considered (accounting for heating and cooling) a maximum efficiency of 96.7% is achieved, and the system CO2 emissions decrease by 33.2%. Similarly,

Wu & Wang (2006) determined that efficiency in polygeneration systems, ranging from 70-90%, improves dramatically when compared to centralized power plants which only have a fuel utilization of 30-45%. There are several studies of small-scale polygeneration systems performed by Katsaprakakis & Voumvoulakis (2018), Karellas & Braimakis (2016), Chua et al. (2014), Calise et al. (2016) and the European Union with the Concerto Programme (2014). However, most of these projects were done on islands and communities in Europe. This section focuses on three specific small-scale polygeneration cases to consider projects that are similar to the case study in this thesis. Two of them are systems developed for hotels and the other is an example of a polygeneration system in a similar climate condition.

First, a look at a polygeneration system for a hotel in the Mediterranean region of Spain is considered. In this system, the prime mover is fuelled by biogas produced by biomass, and a solar collector system is also used. The outputs are electricity, heating, cooling, and purified water from a desalination system, as seen in Figure 10.

Figure 10. System schematic of a polygeneration system in the Mediterranean region in Spain (Rubio-Maya et al. 2011)1

Unfortunately, in this case study, the efficiencies of the system were not identified. However, a comparison was made with a natural gas system to obtain the emissions saved per year, and the results show that

1_{SGAS = Biomass Gasification System, AXB= Auxiliary Boiler, PM = Prime Mover, DHW= Domestic Hot Water,}

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polygeneration systems have huge benefits considering environmental aspects. In this study, the biomass-solar polygeneration system avoided 3,114 tons per year of GHG emissions when compared to the natural gas system (Rubio-Maya et al. 2011).

Another small-scale polygeneration system that is of great interest is a dairy farm of 30 associate families in Bolivia that was studied by Villaroel-Schneider et al. (2020). For the polygeneration system, the outputs are electricity, refrigeration, biogas, and fertilizer production. The polygeneration plant (PP) was studied for two configurations: one with an internally fired microturbine (IFMT) and another with an internal combustion engine (ICE). The production of biogas to power the prime movers is done by an anaerobic biogas digester fed with cow dung and water. In this case study, the production of the outputs is more than the demand needed by the dairy farm so some of the excess products can be sold on the market. Figure 11 shows the cost comparison of biogas, electricity, and cooling for the two polygeneration systems proposed and the prices of the services in the Bolivian market.

Figure 11. Cost comparison of services supplied by the proposed PP's systems versus services in the market (Villaroel-Schneider et al. 2020)

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taken into consideration. Figure 12 illustrates a comparison of the net present cost (NPC), the capital investment and the CO2 emissions for the four different cases.

Figure 12. Comparison of NPC, capital investment and CO2emissions for all four cases (Wegener et al.

2019).

For the NPC, the fourth case is the best option in comparison to the other three cases and the difference from the base case is significant. On the other hand, the capital investment is very low for the first case as the only system component is the back-up diesel generator, while in the other three cases the upfront investment is high because a new system would need to be deployed. Lastly, the comparison of the CO2

emissions shows how beneficial a fossil-fuel-free system can be to the environment. As the authors state, this study indicates enormous ecological and economic potential of biomass-based solutions compared to conventional systems, leading to savings of more than $578,000 over a period of 20 years, a payback period of less than 4 years and saving 365 tons of CO2 per year (Wegener et al. 2019).

There are not many polygeneration studies in the hotel sector, which gives reason for the case study in this thesis to be a valuable resource for future polygeneration systems in this sector. As the United Nations World Tourism Organization (2020) states, the hotel sector is one of the tourism industry’s largest drivers of employment and economic revenue, but at the same time, it is one of the most energy intensive. In fact, hotels, and other types of accommodation account for up to 5% of global CO2 emitted by the tourism

sector. For that reason, the decentralization of power and use of thermal energy for heating, cooling, and water purification (CCHPW) could contribute to the improvement of sustainability in tourism, which is a crucial economic sector, but very insensitive in its energy and water consumption (Rubio-Maya et al. 2011).

3.4 Feasibility of Sustainable projects in Costa Rica

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ecotourism is of great importance in the country, this thesis aims to be developed in a hotel that has ecolodge characteristics and thus, it is apparent that the hotel would be interested in a polygeneration system powered by RES.

Another important parameter that should be mentioned is the natural resources that the country has, which could be exploited in an environmentally friendly way to produce renewable energy. Costa Rica is situated close to the equator, which means that it has 12h of daily solar irradiation all year around. Further, the country also has seven active volcanoes that can be utilized for geothermal energy production, it is surrounded by two oceans in its coasts, it has large portions of its land designated to the agriculture industry, which generates biomass, and it has a very diverse geographical structure, with mountains and rivers, making it ideal for hydropower. However, despite all the different natural resources that the country has, the mix of energy generation is very poor, as seen in previous Figure 2.

Solar energy capacity for Costa Rica will be studied in a later chapter, but it is important to mention that the country is situated in a zone with some of the highest solar capacities in the world (Ram et al. 2019). That means that solar PV has potential to be exploited further in the country. For that reason, the case study in this thesis focuses on the opportunity to design a polygeneration system to be powered by solar and biomass. The share of renewable energy in the generation mix in Costa Rica is one of the highest in the world. As Ebeling (2020) mentions, in the realm of electricity production, currently around 95% of generation comes from renewable energy sources, and a 100% goal is set for 2030. However, as previously mentioned most of that renewable energy is obtained from hydropower. Hydropower plants are, currently a key electricity generator worldwide, especially in tropical countries. With a share of 16% in the world power generation and reaching 56% of the supply of electrical energy demand in Central and South America (TSP, 2018), this source has always been considered a clean way of generating energy. Despite being a renewable energy source, it has been discovered that it is not operated without Greenhouse Gas emissions (GHG).

Figure 13. Carbon cycle in a hydropower reservoir (IHA, 2020)

Large reservoirs produce CO2 and CH4 when microorganisms present in the water decompose organic

matter that is trapped in the flooding area, as explained in Figure 13. The GHG emissions emitted by a hydropower plant in tropical areas are notably higher than in other regions, due to the higher temperatures and higher irradiation, two factors directly linked to an increase in CO2 equivalent emissions. The

International Hydropower Association (IHA, 2018), establishes that hydropower reservoirs have a median life cycle carbon equivalent intensity of 18.5 g CO2 -eq/kWh., in comparison with other sources as coal that

has a carbon equivalent intensity of 820 g CO2 -eq/kWh or gas that has 490 g CO2 -eq/kWh, hydro is a

positive source. But if compare to nuclear that produces 12 g CO2 -eq/kWh or wind and solar PV that have

0 g CO2 -eq/kWh (taking into consideration only generation, not the whole LCA cycle) then the pollution

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Figure 14.GHG emissions intensity (g CO2 -eq/kWh) by climate region (IHA, 2018)

This means that despite the electricity generation mix being almost 100% renewable in Costa Rica, there are some GHG emissions produced by large hydropower plants that need to be considered. This is a main reason why this thesis project focuses the attention on other renewable energies such as solar, wind and biomass to help diversify the generation mix in Costa Rica and decrease the percentage of energy produced by hydropower.

Another important factor that contributes to the feasibility of a polygeneration project for hotels in Costa Rica is the high price of electricity in the country. The monthly cost of electricity for commercial and business starts at $0.23 per kWh when the usage is between 0-3,000 kWh and $0.14 per kWh when the usage exceeds 3,000 kWh.

Figure 15. Price of electricity for commercial and business in Costa Rica (ICE, 2020)

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4 Case Study: Design of Polygeneration System

4.1 HOMER Pro Software

For the simulation of the case study, a software often used by researchers for microgrids will be utilized. Hybrid Optimization Model for Multiple Energy Resources (HOMER) software was originally developed by the National Renewable Energy Laboratory (NREL). This software allows the user to design and optimize distributed energy generation projects, and to evaluate their cost effectiveness and technical implementation (HOMER, 2020). To run the techno-economic optimization simulation HOMER requires six types of input data, which are: meteorological data, load profile, equipment characteristics, search space, economic and technical data (Bahramara et al. 2016). These input data can be introduced by the user or it can be obtained in the software interface. The meteorological data in HOMER is obtained from the NASA surface meteorology and solar energy database, and the equipment characteristics can be obtained from the software database. Figure 17 provides a visualization of the optimization process carried out by HOMER.

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allows the user to compare different simulation solutions and choose the best solution for the case depending on the user’s preferences.

The net present cost (NPC) of a component is the present value of all costs of installing and operating the component over the project timeline, minus the present value of all revenues that it earns over the project lifetime. HOMER calculates the NPC of each component of the system and of the entire system (HOMER, 2020). The equations used by HOMER to calculate the NPC are as follows:

𝑁𝑃𝐶 = 𝐶𝑎𝑛𝑛,𝑡𝑜𝑡𝑎𝑙

𝐶𝑅𝐹 (𝑖,𝑅𝑝𝑟𝑜𝑗) (1)

𝐶𝑅𝐹 (𝑖, 𝑅𝑝𝑟𝑜𝑗) =

𝑖(1+𝑖)𝑅

𝑖(1+𝑖)𝑅₋₁ (2)

𝐶𝑎𝑛𝑛,𝑡𝑜𝑡𝑎𝑙 is the total annualized costs and CRF is the capital recovery factor. The lifetime of the project is

represented by R, and i represents the interest rate for the project.

Another important parameter for system evaluation is the levelized cost of energy (COE). HOMER (2020) defines COE as the average cost per kWh of useful electrical energy produced by the system. The equation used by HOMER to calculate the COE is as follows:

𝐶𝑂𝐸 = 𝐶𝑎𝑛𝑛,𝑡𝑜𝑡𝑎𝑙−(𝐶𝑏𝑜𝑖𝑙𝑒𝑟)(𝐻𝑠𝑒𝑟𝑣𝑒𝑑)

𝐸𝑠𝑒𝑟𝑣𝑒𝑑 (3)

𝐶𝑎𝑛𝑛,𝑡𝑜𝑡𝑎𝑙 is the total annualized costs, 𝐶𝑏𝑜𝑖𝑙𝑒𝑟 is the boiler marginal cost, 𝐻𝑠𝑒𝑟𝑣𝑒𝑑 is the total thermal load

served, and 𝐸𝑠𝑒𝑟𝑣𝑒𝑑 is the total electrical load served.

HOMER is an effective tool for research due to the ease of use of the platform and the multiple results obtained for each simulation, making it convenient for comparison and analysis of how different parameters influence the results.

4.2 Resources in the Region and Ambient Conditions

The building chosen for the case study is a hotel in the Guanacaste province of Costa Rica. The province of Guanacaste has a total extension of 10,140 km2_{, which is 20% of the total territory of Costa Rica (MAG,}

2018). The hotel in the case study is in the Nicoya area of the province, in northwest Costa Rica, as seen in Figure 18.

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The region of study has tropical, dry, and hot weather, with the dry season from December to May and the rainy season from June to November. The maximum mean annual temperature is 33º Celsius and the minimum mean annual temperature is 22º Celsius, with an annual average of 27.5º Celsius. The mean annual precipitation at the region averages 1,652.7 mm. The driest months are January with 3.9 mm, February with 12.0 mm, and March with 5.2 mm of precipitation (IMN, 2018).

4.2.2 Wind resources

Costa Rica is mountainous in the center of the country and flattens along the coasts. This creates a wind passage through the middle of the country while on the coasts the wind resources are not as strong. This is caused by the country`s shape and the fact that the northern neighbor Nicaragua, is more extensive in terrain which creates an effect that slows down much of the incoming wind.

Figure 19. Wind speed in Costa Rica (Global Wind Atlas, 2020)

As seen in Figure 19, the highest wind speed occurs in the middle of the country and in the area of study the average wind speed is only 5 m/s. These conditions are still suitable for installation of wind turbines but are not ideal as the wind is not strong enough to yield high energy outputs.

4.2.3 Solar resources

Costa Rica is situated at only 9.75º North of the equator, with a zone of high solar irradiance. As shown in Figure 20, the highest solar irradiance in the country occurs in the west pacific, where the hotel of study is located.

The yearly solar irradiance in the specific location of study is of 2,048.93 kWh/m2 _{(5.61 kWh/m}2_{/day), with}

the highest radiance values being measured during the dry season from December to May with a monthly average of 200 kWh/m2_{, and during the rainy season the monthly average is of 150 kWh/m}2_(European

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Figure 20. Solar irradiance in Costa Rica (European Commission PVGIS, 2020)

4.2.4 Biomass availability

The main use of the land in the region is for agriculture and cattle raising. As mentioned by the Ministry of Agriculture (MAG, 2018), the land that is not used for residential or commercial purposes, is used as 70.47% for agriculture of grains, fruits, vegetables, and flowers; 22.58% for cattle raising and 6.95% for protected areas and parks.

The agriculture sector uses a total of 85,418 hectares for growing vegetables and fruits such as: rice, onions, peppers, beans, corn, watermelon, tomatoes, avocados, coffee, sugarcane, mangoes, oranges, and other crops. The two main crops in the region are rice and sugar cane, with an estimated 28.46% and 41.85% of the total land use for agriculture in the region respectively (MAG, 2018). This information indicates that biomass is present in the region and could be used as a resource for power generation.

This case study focuses on the use of biomass from the two main crops in the region. The most harvested crop in the region is sugar cane with a total of 35,754 hectares used. The annual production of sugarcane in the region is 60 tons per year per hectare (MAG, 2007). This means that the total annual production of sugarcane is 2,145,240 tons. Of the sugarcane, 69% is used in the production of sugar, 10% is considered waste and 30% is bagasse which is a byproduct that can be used for the generation of syngas for electricity. Using this information as a reference, the total biomass obtained from sugarcane in the region is 643,572 tons per year. Sugarcane bagasse has a Lower Heating Value (LHV) of 7,980 MJ/ton (Shukla & Kumar, 2017).

Rice is the second most harvested crop in the region with 24,313 hectares of land used. As mentioned by the Food and Agriculture Organization of the United Nations (FAO, 2004) the yield of rice is 5.5 tons per year per hectare in Latin America. This adds up to 133,721 tons per year of rice harvested every year in the region. In the harvest of rice, rice straw represents 29% of the total harvest and rice husk represents a 20% of the total harvest, rice straw has an LHV of 10,460 MJ/ton and rice husk as an LHV of 12,550 MJ/ton (Singal et al. 2007).

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the operating pressure. Considering these parameters, the efficiency of the gasifier usually ranges between 65% and 80%. For the calculation in this study, a gasifier efficiency of 75% as calculated by Meyer & Mamphweli (2010) will be assumed. Table 2 shows the energy that could be obtained from the biomass present in the region.

Type of Biomass LHV

(MJ/ton)

Biomass available per year (ton)

Syngas mass available per year

(ton) Syngas energy available for combustion per year (MWh/year)1 Sugarcane Bagasse 7,980 643,572 482,679 1,070,021.05 Rice Husk 12,550 26,744 20,058 69,930.01 Rice Straw 10,460 38,779 29,084 84,512.86 Total --- 709,095 531,821 1,224,463.92 1_{Conversion from MJ to MWh is 0.0002778}

Table 2. Annual Potential for biomass gasification in Chorotega region

The weather and ambient conditions in the Chorotega region, where the hotel of the case study is situated, make these three resources very important in the development of the system. Hydro resources were also considered for the case study, but the hotel regional water flow is not steep enough and the precipitation is not high enough during six months of the year, so hydro resources are neglected.

4.3 System Boundaries and Assumptions of Case Study

As previously mentioned, some assumptions had to be made in order to have a more accurate demand data for the different systems. For the air conditioning system, the demand data was calculated based on the A/C units in the hotel and the power ratio of the units. For the domestic hot water (DHW) demand data, an assumption based on a constant usage for the quantity of people in the hotel was used to calculate how much thermal power is needed to heat up water. Similarly, the energy usage of the MD system was calculated based on the amount of water needed per person in the hotel, as explained in Table 5.

It was assumed that the residual biomass waste is currently not being used for any purpose and hence a part of the waste can be used for syngas production. An assumption was made to calculate the price of the biomass waste. It is important to mention that in this region, biomass energy is not commercially traded, but some large agricultural plants use sugarcane bagasse to power their own electricity plants (El Viejo, 2020). To estimate the price for the biomass that would be used in this case study, a document by the Ministry of Agriculture in Costa Rica (2007) was considered. Using the values in that document, a price for the bagasse produce in a hectare was calculated in Table 3.

Table 3. Price of bagasse per hectare

In the table above, the price was calculated per hectare. As mentioned previously, in one hectare, 18 tons of bagasse are produced, so the price per ton can be calculated, as shown in Table 4. To the cost of the

Cost type ($/hectare)

Labor work 45

Equipment, water, fertilizer, and electricity

155

Marginal 40% 80

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bagasse the transportation cost must be added. After some research for transportation costs in the Guanacaste region, it has been concluded that a truck with a capacity to carry two tons of bagasse charge $60 per trip, which means that the transportation cost per ton of bagasse is $30.

Table 4. Price of bagasse per ton

Including all costs, it has been estimated that for the case study, a ton of sugarcane bagasse has a value of $45.55, this cost is included in the HOMER simulation to have an accurate COE for later comparison. Also, some assumptions needed to be made for the selection of the system components. For the production of syngas, a down draft gasifier was selected due to its proven technology maturity with agricultural biomass, with an efficiency of 75% (Meyer, 2015). An ICE was selected as prime mover due to the maturity of the technology, the high efficiency at small sizes, and higher tolerance to contaminants than turbines, which is especially useful for small-scale biomass applications (Stanek et al. 2015). The ICE electrical efficiency is around 30%, but due to the 2:1 heat to power ratio the overall efficiency can be increased to 90% when utilizing recovered heat for energy service purposes (Mertzis et al. 2014). Another assumption that has been made is the efficiency of the absorption chiller to covert the residual heat into cooling power. The absorption chiller in this case study is set to have an efficiency of 60% (Matjanov, 2020).

4.4 Data Acquisition of Electric and Cooling Demand

The hotel considered for the case study has 26 rooms for double occupancy, a pool, and a restaurant. As seen in Figure 21, the hotel has its highest occupancy from November to April, with March being the month with the highest occupancy. The hotel has a lower demand from May to October, with October showing the lowest occupancy month.

Figure 21. Monthly electrical demand for the hotel in 2019

Cost type ($/ton)

Production cost 15.55

Transportation cost 30

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This data correlates with the dry and rainy season, respectively, and data provided by the tourism institute of Costa Rica (Instituto Costarricense de Turismo), where March has the highest occupancy per month at 82.8% and October has the lowest occupancy per month at only 47.3% (ICT, 2019).

The hotel provided the demand data for the year 2019 for use in this case study. The hotel systems for air conditioning (A/C) and domestic hot water (DHW) are run electrically. The electric demand data includes these systems as well as a pump to extract water from an underground well, the system for the functioning of the pool, and the electrical appliances that are used in the hotel rooms (ceiling fans and power outlets) and in the restaurant (stoves, ceiling fans, microwaves, refrigerators, freezers, TVs, etc.).

The hotel provided data based on monthly electrical usage, therefore it was necessary to generate hourly demand data based on reference studies (Wegener et al. 2019; Smith et al. 2020). The data in Figure 22, presents all 8760 hourly values of electricity demand from the first hour of the year in January to the last hour in December for the year 2019. The blue bars represent hourly electric demand data, which fluctuate depending on the occupancy of the hotel. The red line shows an hourly average, which correlates to the monthly data previously showed in Figure 21.

Figure 22. Electrical hourly data for the full year in 2019

In this case study, the A/C and the DHW outputs are going to be obtained from a heat recovery system using a biomass genset and a boiler. To obtain the demand data for these two systems from the provided electrical data some assumptions were made. It was investigated that in the hotel a total of 30 A/C units with an output power of 1.4 kW are used. This means that if the occupancy of the hotel was 100% then the A/C will consume 42 kWh/hour and if the hotel was 50% then only 24 kWh/hour of electrical output will be needed. As for the DHW it was identified that assuming a constant production of 20 l/occupant/day for 75 persons the hot water demand will use 4.39 kW at full occupancy (Aichmayer et al. 2014).

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Figure 23. Demand profile for March 2019

In Figure 24, the demand profile for the lowest month of the year is presented. The variations during daily use does not change much, but the intensity is decreased due to the low occupancy of the hotel for all energy service appliances.

Figure 24. Demand profile for October 2019

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capita (l/person/day)

Total water demand (l/day)

Total water demand (m3_/day)2

52 20 1040 1.04

2_{Coversion from liter to m}3_{is .001}

Table 5. Water demand estimation for the hotel

As mentioned before, the MD system utilizes thermal energy to purify the water. Ullah et al. (2008), calculated that the thermal energy required by a MD system is 100 kWh/m3_{. This means that for the hotel}

in this case study, the total heat energy required to power the system is 104 kWh per day. This thermal demand for the MD system fluctuates during the day to account for the hours that more purified water is needed.

To account for the MD system as well as the DHW and A/C systems to be driven by thermal energy, the demand data for the hotel varies. For the case study, there are two demand loads: one electrical demand and one thermal demand. Figure 25 and Figure 26 present the demand loads for March and October respectively.

Figure 25. Energy demand of the hotel in March

Figure 26. Energy demand of the hotel in October

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Figure 27. Demand data for the hotel in the case study

It is important to also mention that, for this case study, the COE from the grid was determined from the electricity bills of the hotel, as explained in Figure 15, the electricity price for hotels in Costa Rica has a fixed price for the first 3,000 kWh and after that, the rate is lower. This means that the COE changes monthly depending on the usage, for the hotel in the case study the highest COE is in the month of October where the occupation is the lowest with a price of $0.22/kWh and the lowest in March when the occupancy is the most, with a price of $0.17/kWh. For the year 2019, the average monthly COE is $0.197kWh, this is the value used in the HOMER simulation.

4.5 Proposed System

The proposed polygeneration system consists of photovoltaic panels, a biomass gasifier to power an ICE, a battery bank to store excess energy, a biomass boiler to help with heating and potable water production, an absorption chiller to generate cooling form heating and a membrane distillation unit to purify water. The system design is shown in Figure 28.

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As seen in the system schematic above, the biomass is fed into the gasifier to produce syngas to be used to fuel the ICE. The ICE and the PV panels produce electrical energy to cover the electrical load. The electrical load in the hotel operates with alternate current (AC), but the solar PV panels produce direct current (DC) electricity. So, the electricity produced by the PV panels in DC is stored in the battery bank, and when it is needed to cover the electrical demand, a converter converts the current from DC to AC. For the AC system, the residual heat from the ICE is converted into cooling power by the absorption chiller. The residual heat is also used to heat up the water to produce DHW in the hotel and using a membrane distillation system the heat is used to produce clean water. To help in the production of thermal energy in case of an energy shortage, a boiler is added as part of the system. The boiler is relatively inexpensive, powered by biomass, and it operates only when needed so it is more an auxiliary part of the system. It is also important to mention that the boiler is added to the system party due to the modelling limitation in HOMER to account for thermal energy.

4.6 Description of System Components

This chapter provides a summary of all the components in the proposed system. It also presents the characteristics of all the components, their capital costs, and the operation and maintenance (O&M) costs. For the prices, scientific and commercial sources have been used for each component, as well as the prices suggested by HOMER Pro. The estimated prices are conservative, in order to account for shipping and engineering costs.

Component

Capital &

Replacement

Cost

O&M Cost

Characteristics

ICE + Down-draft

gasifier

1,060 $/kW (Bhattacharjee & Dey, 2014) 0.1 $/operating h (Sigarchian et al. 2015) Lifetime: 15,000 h (Bhattacharjee & Dey, 2014)

Gasifier efficiency: 75% (Meyer, 2015) Engine max. el. efficiency:

30%

Engine max. efficiency with heat recovery: 90% (Mertzis et al. 2014)

PV panels

1,000 $/kW (Ossenbrink et al. 2012) 25 $/kW/year (Yoo et al. 2014) Lifetime: 25 years Derating factor: 80% Max Efficiency: 15% (Wegener et al. 2019)

Batteries

1,500 $/kWh (Wegener et al. 2019) 30 $/year/battery (Wegener et al. 2019)

Lead Acid type Minimum lifetime: 7 years Lifetime Throughput: 10,973

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Converter

750 $/kW (Sigarchian et al. 2015) 10 $/kW/year (Wegener et al. 2019) Lifetime: 20 years Inverter efficiency: 90% Rectifier efficiency: 90% (Sigarchian et al. 2015)

Boiler

700 $/kW (HOMER database, 2020)

Negligible Boiler efficiency: 85% (HOMER database, 2020)

Absorption Chiller

50 $/ kW (Coronado et al. 2011) .005 $/kWh (Coronado et al. 2011)

Heat recovery rate: 70% COP: 0.6 (Wegener et al. 2019)

Membrane

Distillation

1,080 $/m3 (Perves et al. 2016) 0.2 $/m3 (Perves et al. 2016)

Direct Contact Membrane Distillation Lifetime: 25 years Power ratio: 100 kWh/m3

(Ullah et al. 2008)

CCHP measures

15,000 $/per system

(Wegener et al. 2019)

.013 $/kWh (Coronado et al.

2011)

Capital cost including piping, engineering, and shipment

Lifetime: 25 years Table 6. Summary of system component costs and characteristics

4.7 Load Control of the System

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5 Results and Sustainability Assessment

5.1 Simulation Results

The proposed system schematic is shown in Figure 28 and the system component sizes are displayed in Table 7. The results obtained by HOMER show that the proposed system’s ICE has a configuration that is 10 times larger than the PV system, this is because biomass is used the majority of time and the PV system for peak hours during the daytime.

Component Optimal Configuration

ICE 100 kW

PV 10 kW

Batteries 46 kWh

Converter 10.7 kW

Table 7. Proposed system component sizes

The results obtained by HOMER for the proposed system configuration are compared to usage of electricity from the grid over a period of 25 years. As previously mentioned, HOMER displays the best solution according to the NPC. For the grid case simulation only the price of electricity and the total demand of the hotel were used as input into the simulation. The price of the electricity from the grid was calculated based on the information in section 4.4. No cost was added for grid connection since the hotel is already connected to the grid. After simulating the proposed system case (Figure 28) and the grid system in HOMER, the results are presented in Figure 29.

Figure 29. Economic results

Small Scale Polygeneration System for Hotels in Costa Rica