An Application of Solar Energy Storage in the Gas ...

Energy Sources, Part A, 29:1513–1520, 2007 Copyright © Taylor & Francis Group, LLC ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/00908310600626598

An Application of Solar Energy Storage in the Gas: Solar Heated Biogas Plants G. KOCAR A. ERYASAR Solar Energy Institute Ege University Izmir, Turkey Abstract Temperature is an important factor that may affect the performance of anaerobic digestion. Therefore, biogas plants without heating system work only in warmer regions for the whole year. In regions with extreme temperature variations, for instance in Turkey, the biogas plant should be built with heating system. One of the methods is to use solar energy to increase the reactor temperature. In this study, solar heated biogas plants were reviewed. Furthermore, the optimization of insulation thicknesses and solar energy systems for 5 m3 biogas reactor were carried out for two different cities for three different climatic zones in Turkey. Based on the obtained results, the ratio of annually produced biogas used for reactor heating was calculated for each city, with and without solar heating system. Obtained results indicate that the biogas consumption for reactor heating is decreased by approximately 19% for average of six cities when solar heating system is used. This means that available biogas potential would be increased. Keywords anaerobic digestion, anaerobic reactor, biogas, biogas energy, biomass, energy, solar heated

Introduction Around the world, pollution of the air and water from municipal, industrial and agricultural operations continues to grow. The implementation of biogas technology has a great potential of mitigating several problems related to ecological imbalance, minimize crucial fuel demand, improve hygiene and health, and, therefore, there is an overall improvement in quality of life in rural and semi-urban areas (Rahman et al., 1996). The anaerobic digestion process has a key role in environmental pollution control: methane is an important greenhouse gas, but if captured for use, it acts instead as a good renewable energy source (McCarty, 2001). The rate and efficiency of the anaerobic digestion process is dependent on the following factors (Anonymous, 1999a; Dennis and Burke, 2001): Substrate temperature, available nutrients, retention time, pH level, nitrogen inhibition and C/N ratio, substrate solid content and agitation, inhibitory factors. Temperature is an important factor that may affect the performance of anaerobic digestion (El-Mashad et al., 2004). The amount Address correspondence to Dr. Gunnur Kocar, Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkey. E-mail: [email protected]

1513

1514

G. Kocar and A. Eryasar

of gas produced increases with digester temperature, with retention time and with the percentage of total solid in the slurry. Typically for 25ı C to 44ıC, 0.25 m3 to 0.40 m3 of methane gas is produced for each kilogram of volatile solids destroyed (Jenangi, 1998). Anaerobic digestion can take place at any temperature between 3ı C and 70ıC. Differentiation is generally made between three temperature ranges: The psychrophilic temperature range lies below 20ıC, the mesophilic temperature range between 20ı C and 40ıC, and the thermophilic temperature range above 40ıC (Anonymous, 1999a). Conventional anaerobic digesters are commonly designed to operate in either the mesophilic temperature range or thermophilic temperature range. There are usually two reasons why the mesophilic and thermophilic temperatures are preferred. First, a higher loading rate of organic materials can be processed and, because a shorter hydraulic retention time is associated with higher temperatures, increased outputs for a given digester capacity results. Second, higher temperatures increase the destruction of pathogens present in raw manure (Lusk, 1998). Thermophilic digestion is much faster than mesophilic. This means that thermophilic digesters would be only up to 30% of the size of mesophilic digesters (Zupancic and Ros, 2003). On the other hand, thermophilic digestion demands higher energy input compared to mesophilic digestion (El-Mashad et al., 2004). Most digesters operate in the mesophilic temperature regime. Some operate in the thermophilic regime (Kartha and Larson, 2000). In Europe, more than 85% are operated at the mesophilic temperature range, 8% at thermophilic and 5% at psychrophilic levels (Jorgensen and Van Djik, 2003). To keep the anaerobic digester temperature constant, external source of heating is used like electricity, oil, or part of the produced biogas. The use of such fuel is uneconomical (El-Mashad et al., 2004). Generally, 20–40% of produced biogas is used for digester heating (Sarapatka, 1993; Sasse, 1988; Anonymous, 2004; CAEEDAC, 1999; Demuynck et al., 1984). One of the methods is to use solar energy to increase the digester temperature. This source is environmentally friendly and economical. The present article aims to review solar heated biogas plants. Furthermore, 5 m3 solar heated anaerobic digester was analyzed for six different selected climatic zones in Turkey.

Potential of Biomass and Biogas The term “biomass” means any plant-derived organic matter available on a renewable basis, including dedicated energy crops and trees, agricultural food and feed crops, agricultural crop wastes and residues, wood wastes and residues, aquatic plants and animal wastes (Anonymous, 2002). Globally, photosythesis stores energy in biomass at a rate roughly ten times the present rate of global energy use. However, today less than 1.5% of this biomass is used for energy—estimated to be between 40 to 50 exajoules per year. Biomass is mainly used as a traditional fuel (e.g., fuelwood, dung), contributing to about 38 ˙ 10 EJ/year, and modern biomass to about 7 EJ/year (Hoogwijk et al., 2003). It is estimated that in its widest sense, biomass accounts for a minimum of 15% of total world energy supply, and that within some developing countries biomass accounts for between 35–50% of domestic energy supply. On average, biomass accounts for 3 or 4% of total energy use in industrialized countries, although in countries with policies that support biomass use (e.g., Sweden, Finland, and Austria), the biomass contribution reaches 15 to 20% (Anonymous, 2002). The available amount of dung depends on the number of animals and the requirement of manure as fertilizer. The total amount of manure produced annually is assessed at 46 EJ/year (Hoogwijk et al., 2003). Several studies have assumed that 12.5 to 30% of the total available manure can be recovered for energy production. Hence, the availability of energy from animal manure is approximately 6–14 EJ/year.

Solar Energy Storage in Gas

1515

Worldwide cattle stock is estimated about 1,334,501,290 head (FAO, 2004). Their biogas potential is approximately 350 billion m3 /year. Assuming that lower heating value of biogas is 22.7 MJ/m3 , the worldwide energy equivalent of cattle dung for producing biogas is found to be 2.37 EJ/year. Total biomass potential of Turkey is estimated about 16.9–32 million ton oil equivalent (Mtoe) (Demirbas and Bakis, 2004). The average majority of biomass energy is produced from wood and wood wastes (64%), followed by municipal solid waste (24%), agricultural waste (5%), and landfill gases (5%). Biogas has a potential between 2.2–3.9 billion m3 per year, corresponding to 1–2 million toe provided that all dung is used for biogas production. Around 85% of the total biogas potential is from dung gas, and the remainder is from landfill gas (Ediger and Kentel, 2004). There are 9,800,000 cattle in Turkey (FAO, 2004). Their biogas potential is about 2–2.5 billion m3 /year.

Heating Systems of Biogas Reactors The daily biogas production is dependent on the interaction of several factors which affect methanogenesis, i.e., climatic, biological, and operational conditions (Sarapatka, 1993). Temperature is an important factor that may affect the performance of anaerobic digestion (El-Mashad et al., 2004). The mesophilic temperature regime, the biogas production takes place through anaerobic digestion of cattle dung, water hyacinth and other digestible biomass at the optimum slurry temperature of 35ı C–37ıC (Tiwari et al., 1989). This temperature range is too high to be achieved naturally during the winter months at cold places. In order to achieve the optimum temperature for good production of biogas, external thermal energy is required for the slurry in the digester (Tiwari et al., 1992). Biogas plants without heating system work, therefore, only in warmer regions for the whole year. In regions with extreme temperature variations, for instance in Turkey (hot summer, cold winter), the incorporation of a heating system and control of temperature is necessary (Anonymous, 1999a; Mısra et al., 1992). The heating system is a critical part of the anaerobic digester. In order to heat the influent and to maintain the reactor content at the required temperature, various types of heating devices are as follows: A heat exchanger that heats the reactor content directly when it is located inside the reactor, by recirculation when it is located outside the reactor. This device is more convenient for wastes with a very low solid content. Heating coils or heating draft tubes located inside the reactor, in which warm water is circulated to heat the reactor content. Jacketed vessels that use hot water surrounding the digestion tank. This device is generally applied to pilot-scale anaerobic reactor (Demuynck et al., 1984; Alkhamis, 2000). The heat is supplied by an external source. It is achieved by using one of the following methods:

Utilizing the heat derived by burning part of the biogas output or fossil fuel; Using the exhaust heat from a biogas driven engine; Electrical heating; Using other renewable energy sources (Tiwari et al., 1996).

Electricity, fossil oil, or part of the biogas produced from the process is used to keep the reactor at the desired temperature. The use of such fuels leads to excessive

1516


heating costs, thus making their usage uneconomical (Axaopoulos et al., 2000). One of the methods is to use solar energy to increase the reactor temperature. Heating a biogas reactor via solar energy seemed to be an excellent approach for bioenergy production. Generally, an active solar water heating system consists mainly of solar panels, storage tank, piping between panels and the tank, and a circulation pump plus a differential thermostat if the system is of a forced circulation type. Produced hot water from solar system is used to heat the reactor. Utilization of solar energy for biogas reactor heating has been demonstrated in a number of projects in developing countries (Beba, 1988). Furthermore, several studies were published for utilization of solar heated biogas reactors (Tiwari and Chandra, 1986; Subramanyam, 1989; El-Mashad et al., 2003; Tiwari et al., 1988; Anonymous, 1999b; Padav et al., 1987; Gupta et al., 1988; Axaopoulos and Panagakis, 2003).

An Illustrative Example of Turkey Turkey’s geographical location is highly favorable for utilization of solar energy. Turkey lies in a sunny belt between 36ı and 42ı latitudes. The yearly average solar radiation is 3.6 kWh/(m2 day) and the total yearly radiation period is approximately 2,460 h, which is sufficient to provide adequate energy for solar thermal applications. The gross solar potential of Turkey is calculated as 88 billion toe per year, of which 40% can be used economically (Demirbas, 2001). Total annual production capacity of solar thermal collectors in Turkey has reached about 200,000 m2 . It is also estimated that the amount of the collectors mounted are over 3.5 million m2 . In this sector, there are over 100 companies and 2,000 employees (Hepbasli et al., 2004). Solar collectors are low cost and high efficiency in Turkey. Hence, using solar energy to heat the biogas reactor is a good alternative for Turkey. In this sense, the pilot-scale solar heated anaerobic reactor was constructed at the Solar Energy Institute in Ege University, Izmir (latitude 38.24 ı N, longitude 27.50 ı E), Turkey (Kocar and Eryasar, 2004). Diluted cow and chicken manure were used in this biogas plant. The experiment was carried out using 280 l working volume cylindrical batch-type mixed anaerobic reactor maintained at the optimal mesophilic temperature range (37ıC). The heating required for the reactor was performed using a solar collector combined with a heat exchanger. The reactor was heated by hot water recirculation through a water jacket surrounding the reactors. The solar collector consists of a cupper flat plate of 204 99.5 cm dimensions tilted at 45ı and facing south. The heat exchanger consists of a cylindrical solar storage tank of 47 cm in diameter and 140 cm in length. The solar storage tank and reactor are wrapped with glass wool of 5 cm thickness and aluminum plate of 0.7 mm. The solar storage tank has an electric immersion element of 2 kW for heating water in the event of low solar insolation. The digesters were intermittently stirred by open impeller centrifugal pump. Actual performance studies were carried out from November 2002 to December 2004. All systems are shown in Figure 1. Also, the heating balance of solar-heated mesophilic batch-type anaerobic reactor was studied. Furthermore, based on the results obtained from this study the optimization of 5 m3 anaerobic reactor insulation thickness was carried out for two different cities per three different climatic zones in Turkey (Eryasar and Kocar, 2004). These cities and climatic zones are shown in Table 1. The optimization of solar energy systems for 5 m3 anaerobic reactor heating were carried out for same cities. The life cycle cost analysis employed in this study computes the heating cost over the lifetime of the biogas plant. In theoretical analysis, daily cli-


1517

Figure 1. The solar heated biogas system.

Table 1 The cities used in the model and their climatic zones Climatic zone

Cities

1. Climatic Zone 2. Climatic Zone 3. Climatic Zone

Antalya, Izmir Diyarbakir, Samsun Ankara, Erzurum

matical data of each city concerning to years 2003–2004 received from Turkish State Meteorological Service was used. Obtained insulation thicknesses and solar energy systems for all cities are shown in Table 2. Figure 2 shows the rate of produced biogas used for reactor heating per cities annually, with and without solar heating. The example of Erzurum is very conspicuous. Biogas consumption for reactor heating is decreased about 28%. This rate of decrease of biogas consumption for reactor heating occurred about 19% for average of six cities. These results are indicated that increasing of available biogas potential.

Conclusions The results obtained show that solar energy is a highly desirable alternative for biogas reactor heating in Turkey. The incorporation of solar energy in biogas plants increases the available potential of biogas. In addition, these applications are environmentally friendly. Hence, the mentioned systems seem to be an attractive economic investment.

1518

G. Kocar and A. Eryasar Table 2 Obtained insulation thicknesses and solar energy systems Cities Antalya ˙Izmir Diyarbakir Samsun Ankara Erzurum

Insulation thickness 12 13 13 14 14 16

cm cm cm cm cm cm

Solar energy system 5 5 6 7 6 9

collectors–350 l collectors–350 l collectors–350 l collectors–350 l collectors–350 l collectors–600 l

Hot Hot Hot Hot Hot Hot

water water water water water water

storage storage storage storage storage storage

tank tank tank tank tank tank

Figure 2. The rate of produced biogas used for reactor heating annually: Solar heated reactor; Non-solar heated reactor.

References Alkhamis, T. M., El-khazali, R., Kablan, M. M., and Alhusein, M. A. 2000. Heating of a biogas reactor using a solar energy system with temperature control unit. Solar Energy 69:239–247. Anonymous. 1999a. Biogas digest—Biogas basics Vol 1. ISAT (Information and Advisory Service on Appropriate Technology). http://www5.gtz.de/gate/ Anonymous. 1999b. Biogas-solar energy integrated utilization project. Agrometeorology Institute. Chinese Academy of Agricultural Sciences. Anonymous. 2002. A centre of excellence for renewable energy in the east of England. A report by Douglas-Westwood Limited for the East of England Development Agency. Anonymous. 2004. Waste treatment systems. GBU. Company for environmental technology.


1519

Axaopoulos, P., Panagakis, P., Tsavdaris, A., and Georgakakis, D. 2000. Simulation and experimental performance of a solar-heated anaerobic digester. Solar Energy 70:155–164. Axaopoulos, P., and Panagakis, P. 2003. Energy and economic analysis of biogas heated livestock buildings. Biomass and Bioenergy 24:239–248. Beba, A. 1988. Analysis of a solar-heated biogas fermenter. Solar Energy 40:281–287. CAEEDAC (The Canadian Agricultural Energy End Use Data and Analysis Centre). 1999. The economics of biogas in the hog industry. www.usask.ca/agriculture/caedac/PDF/HOGS.pdf Demirbas, A., and Bakıs, R. 2004. Energy from renewable sources in Turkey: Status and future direction. Energy Sources 26:473–484. Demirba¸s, A. 2001. Biomass and the other renewable and sustainable energy options for Turkey in the twenty-first century. Energy Sources 23:177–187. Demuynck, M., Nyns, J., and Palz, W. 1984. Biogas Plants in Europe. Energy from Biomass Series. 6. Boston, USA: D. Reidel Publishing Company. Dennis A., and Burke, P. E. 2001. Dairy waste anaerobic digestion handbook. www.makingenergy .com Ediger, V. S., and Kentel, E. 2004. Renewable energy potential as an alternative to fossil fuels in Turkey. Energy Conversion and Management 40:743–755. El-Mashad, H. M., van Loon, Wilko, K. P., Zeeman, G., Bot, P. A. Gerard, and Lettinga, Gatze. 2004. Design of a solar thermophilic anaerobic reactor for small farms. Biosystems Engineering 87:79–87. El-Mashad, H. M., van Loon, Wilko, K. P., and Zeeman, G. 2003. A model of solar energy utilisation in the anaerobic digestion of cattle manure. Biosystems Engineering 84:231–238. Eryasar, A., and Kocar, G. 2004. The practicability of solar heated biogas reactors in Turkey conditions. Bioenergy Symposium 2004. Ege University, Izmir. FAO. 2004. http://faostat.fao.org/faostat/ Gupta, R. A., Rai, S. N., and Tiwari, G. N. 1988. An improved solar assisted biogas plant (fixed dome type): A transient analysis. Energy Convers. Mgmt. 28:53–57. Hepbasli, A., Ulgen, K., and Eke, R. 2004. Solar energy applications in Turkey. Energy Sources 26:551–561. Hoogwijk, M., Faaij, A. P. C., van den Broek, R., Berndes, G., Gielen, D., and Turkenburg, W. C. 2003. Exploration of the ranges of the global potential of biomass for energy. Biomass & Bioenergy 25:119–133. Jenangi, L. 1998. Producing methane gas from effluent. www.ees.adelaide.edu.au/pharris/biogas/ project.pdf Jorgensen, K., and Van Djik, A. 2003. Overview of biomass for power generation in Europe. http://www.ec-asean-greenippnetwork.net/documents/tobedownloaded/ knowledgemaps/KM_ overview_biomass_power_Europe.pdf Kartha, S., and Larson, E. D. 2000. Bioenergy primer modernised biomass energy for sustainable development. United Nations Publications. Kocar, G., and Eryasar, A. 2004. Anaerobic Batch Digestion of Various Organic Wastes. E.U. Research Project, No: 2000/GEE/002. Lusk, P. 1998. Methane recovery from animal manures: The current opportunities casebook. NREL. McCarty, P. L. 2001. Water technology development in the twenty-first century. http://www7. nationalacademies.org/wstb/12001%20Wolman%20Lecture.pdf Mısra, U., Singh, S., Singht, A., and Pandey, G. N. 1992. A new temperature controlled digester for anaerobic digestion for biogas production. Energy Convers. Mgmt. 33:983–986. Padav, Y., Tiwari, G. N., Rai, S. N., and Chandra, A. 1987. An improved solar assisted biogas plant: A transient analysis. Energy Convers. Mgmt. 27:153–157. Rahman, M., Mottalib, M., and Bhuiyan, M. 1996. A study on biogas technology in Bangladesh. 22nd WEDC Conference. Reaching the Unreached: Challenges for the 21st Century. New Delhi, India, pp. 339–641. Sarapatka, B. 1993. A study of biogas production during anaerobic fermentation of farmyard manure. Biomass and Bioenergy 5:387–393.

1520


Sasse, L. 1988. Biogas plants. http://www5.gtz.de/gate/ Subramanyam, S. 1989. Use of solar heat to upgrade biogas plant performance. Energy Convers. Mgmt. 29:73–75. Tiwari, G. N., and Chandra, A. 1986. A solar-assisted biogas system: A new approach. Energy Convers. Mgmt. 26:147–150. Tiwari, G. N., J. Usmani, A. and Chandra, A. 1996. Determination of period for biogas production. Energy Convers. Mgmt. 37:199–203. Tiwari, G. N., Singh, S. K., and Thakure, K. 1992. Design criteria for an active biogas plant. Energy 17:955–958. Tiwari, G. N., Chandra, A., Singh, K. K., Sucheta, S. Y., and Yadav, P. 1989. Studies of KVIC biogas system coupled with flat plate collector. Energy Convers. Mgmt. 29:253–257. Tiwari, G. N., Rawat, D. K., and Chandra, A. 1988. A simple analysis of conventional biogas plant. Energy Convers. Mgmt. 28:1–4. Zupancic, G. D., and Ros, M. 2003. Heat and energy requirements in thermophilic anaerobic sludge digestion. Renewable Energy 28:2255–2267.