Solar assisted biogas plants III: Energy balance of fixed dome biogas plants and enhancement of production in winters

This paper presents a thermal model of a fixed dome biogas plant, with or without hot charging and having the ground surface above the plant, blackened and glazed. The rate of energy transfer, between the ground and the plant has been determined by electrical simulation experiments, using a small scale model of the plant (made of copper), suspended in copper sulphate solution. For hot charging a shallow solar pond, built on the ground has been considered. The monthly variation of the slurry temperature, corresponding to 1, 4, 8, 15 and 30 m³ plants have been evaluated for Madras, New Delhi and Srinagar, which correspond to typical hot, composite and cold climates; the following cases have been considered:

• 1 Bare plant, without hot charging and blackening/glazing of the ground, above the plant.
• 2 Plant, charged with hot slurry from a shallow solar pond.
• 3 Plant, with ground above blackened and glazed.
• 4 Plant, with ground above blackened and double glazed and also provision for heating the slurry by burning a part of the produced biogas.

It is seen that blackening and single glazing of the ground above the plant is able to maintain the slurry temperature between 28 and 35°C throughout the year in composite climate such as Delhi; in cold climates even double glazing alone is not enough for that purpose. Hot charging from a shallow solar pond alone is not sufficient to maintain satisfactory slurry temperatures in winters of Delhi and of course Srinagar. In hot climates the slurry temperature is sufficiently high, except for a couple of months, when either hot charging from a shallow solar pond or blackening/glazing of the ground above is sufficient to have the slurry temperature in the desirable range. In cold climates such as Srinagar, the desired slurry temperatures can be maintained if a part of the gas produced is burnt for heating the slurry, in addition to blackening and double glazing of the ground above the dome. Theoretical predictions of slurry temperatures have been compared with the field data recorded for an 8 m³ digester at the Energy Complex, Masoodpur, Delhi; the agreement is satisfactory.     

Solar assisted biogas plant IIA: Experimental validation of the mathematical models for floating drum type biogas plant

The theoretical predictions presented in an earlier paper for the performance of solar assisted biogas plants are compared with experimental measurements made on four separate systems. There is good agreement between the results and the theoretical predictions.     

Solar assisted biogas plants IV: Optimum area for blackening and double glazing over a fixed-dome biogas plant

The paper presents the economic analysis of a fixed-dome biogas plant of rated capacity 8 m³, above which a part of the ground is blackened and doubly glazed in the cold climate of Srinagar. Blackening and glazing of the ground cannot alone maintain the slurry temperature at 35°C, which is the optimum temperature in the mesophilic range for the anaerobic digestion of cattle dung, and so a part of the biogas must be burnt. The electrical simulation experiments have been performed to determine the loss or gain of heat from the underground biodigestor to the ambient atmosphere through the ground if a part of the ground above is blackened and double glazed. Economic analysis of the system shows that the optimum area to be blackened and glazed would have a radius 1.5 times that of the biodigestor.     

Solar assisted biogas digesters I: Thermal analysis

A thermal analysis has been made of a number of viable solar systems, namely (i) a solar canopy, (ii) a shallow solar pond (SSP) water heater, (iii) insulation and (iv) an SSP with a solar canopy around the digester, to boost the biogas production in conventional KVIC biogas plants during the winter months. Numerical calculations for a typical sunny winter day (17 January 1984) in the climate of New Delhi show that an SSP water heater on the top of the gas holder, covered with a solar canopy, is a good option from thermal point of view.     

Solar assisted biogas plants IV A: Experimental validation of a numerical model for slurry temperature in a glazed fixed-dome biogas plant

The paper presents a comparison between the theoretical and experimental temperatures of digester slurry in a glazed fixed-dome biogas plants. The agreement is within ±2.6°C except for an instance of 4.2°C in the month of April 1988.     

A study of a greenhouse concept on conventional biogas systems for enhancing biogas yield in winter months

A new concept is introduced of using a greenhouse for enhancing the biogas yield from a conventional biogas system in the winter months. If a conventional biogas system is glazed, the trapped solar energy can be used to raise the temperature of the slurry which normally goes low enough to reduce the gas yield appreciably. Numerical calculations have been performed corresponding to the meteorological data on a typical winter day, i.e. 19 January 1981, at New Delhi (India). A comparative study of the performances of conventional and solar-assisted biogas plants using the concept of a greenhouse indicates that the ambient temperature of the slurry can be raised from 18°C to about 37°C, the optimal temperature for anaerobic fermentation.     

Biomass and biogas energy in Thailand: Potential, opportunity and barriers

Biomass has been traditional energy source in rural Thailand for decades. Country modernization, instead of reducing the biomass energy consumption, has continuously increased its utilization for both households and production of modern energy. While the GDP/capita was triple during 1980–1996, the biomass energy consumption increased by 68%, and expected to be higher as signaled by the increasing number of new biomass power plants. The legal and financial support frameworks to increase renewable energy utilization was launched in 1992 aiming for the country to depend less on imported energy, which at present accounted for 7% of GDP. In this paper, biomass and biogas energy situation in Thailand is presented. Resource potential, the promotion program and its success and failure were analyzed to suggest new policy option recommendation. The amount of agricultural residues is about 61 million ton a year, of which 41 million ton, which is equivalent to about 426 PJ of energy, was unused. The most promising residues are rice husk, bagasse, oil palm residue and rubber wood residue, merely due to their availability at the mills, which heat–power cogeneration is feasible. Biogas resources are from industrial wastewater and live stocks manure, which have potential of 7800 and 13,000 TJ/y, respectively. Instead of having the legal and financial support frameworks, the success of biomass energy program in Thailand is doubtful. Discussion on mechanisms and measures based on the past policy, which has been implemented for over a decade, is presented. From lessons learned we propose new policy approach to, in our opinion, overcome the biomass energy utilization barrier in Thailand.     

Enhancement in daily production of biogas system

In this communication, a straightforward analysis of a conventional biogas system (KVIC) integrated with a greenhouse at the top of the dome has been presented. The effect of the use of movable insulation during off-sunshine hours has been included in the analysis. The use of night insulation reduces the heat losses from the top of the system, which causes an appreciable increase in the production of the biogas system. Numerical calculations have been made for a typical winter day in Delhi.     

Energy and environmental balance of biogas for dual-fuel mobile applications

Considerable research is currently being devoted to seeking alternative fuels to comply with transportation needs while reducing the environmental impact of this sector. Within the transport activity sector, on road vehicles and agricultural machinery require around 2 Mtoe energy in France. The anaerobic digestion of farm waste could roughly cover these needs. This paper aims to study the environmental and energy interest of this short power supply path. An ideal biogas production system has been built up from the average characteristics of current rural biogas plants in France. Pollutant emissions, energy demands and production are assessed for various scenarios in order to produce methane for dual fuel engines. Life cycle assessment (LCA) is used to evaluate the environmental impact of dual fuel agricultural machines, compared to diesel engines. The energy balance is always in disfavour of biogas fuel, whereas LCA energy indicators indicate a benefit for biogas production. This gap is related to the way in which the input of biomass energy is handled: in conventional biofuel LCA, this energy is not taken into account. A carbon balance is then presented to discuss the impact of biogas on climate change. Dual fuel engines were found to be interesting for their small impact. We also show, however, how the biogenic carbon assumption and the choice of allocation for the avoided methane emissions of anaerobic digestion are crucial in quantifying CO₂ savings. Other environmental issues of biogas fuel were examined. Results indicate that are management and green electricity are the key points for a sustainable biogas fuel. It is concluded that biofuel environmental damage is reduced if energy needs during biofuel production are covered by the production process itself. As agricultural equipment is used during the biofuel production process, this implies that a high substitution rate should be used for this equipment.     

Solar assisted heat pump on air collectors: A simulation tool

The heating system of the bioclimatic building of the Greek National Centre for Renewable Energy Sources (CRES) comprises two heating plants: the first one includes an air source heat pump, Solar Air Collectors (SACs) and a heat distribution system (comprising a fan coil unit network); the second one is, mainly, a geothermal heat pump unit to cover the ground floor thermal needs. The SAC configuration as well as the fraction of the building heating load covered by the heating plant are assessed in two operation modes; the direct (hot air from the collectors is supplied directly to the heated space) and the indirect mode (warm air from the SAC or its mixture with ambient air is not supplied directly to the heated space but indirectly into the evaporator of the air source heat pump). The technique of the indirect mode of heating aims at maximizing the efficiency of the SAC, saving electrical power consumed by the compressor of the heat pump, and therefore, at optimizing the coefficient of performance (COP) of the heat pump due to the increased intake of ambient thermal energy by means of the SAC. Results are given for three research objectives: assessment of the heat pump efficiency whether in direct or indirect heating mode; Assessment of the overall heating plant efficiency on a daily or hourly basis; Assessment of the credibility of the suggested simulation model TSAGAIR by comparing its results with the TRNSYS ones.     

Heating of a biogas reactor using a solar energy system with temperature control unit

Maintaining a temperature level of about 40°C, the optimum temperature for mesophilic bacteria’s growth and activity, is crucial for obtaining the best results with respect to biogas production. This study investigates the utilization of solar energy for heating a bioreactor that was already developed at our laboratory and is currently in operation. For purposes of this study, a solar collector combined with a heat exchanger are designed, manufactured, and installed to provide the necessary heating requirements for a water jacket around the bioreactor. In addition, an inexpensive PID (proportional. integral, and differential) controller was also designed, installed, and tested to maintain a constant temperature of 40°C in the water-jacket around the bioreactor. The theoretical efficiency of the solar collector was computed and found to be about 61%. The controller action was seen to be very effective and a prompt response for little variation in the temperature of the water jacket (ΔT=1 K) was noticed. Moreover, the internal rate of return of the investment in such solar heating system was found to be 32.7%, which indicates that we have an attractive investment.     

Local acceptance of existing biogas plants in Switzerland

After the Swiss government's decision to decommission its five nuclear power plants by 2035, energy production from wind, biomass, biogas and photovoltaic is expected to increase significantly. Due to its many aspects of a direct democracy, high levels of public acceptance are necessary if a substantial increase in new renewable energy power plants is to be achieved in Switzerland. A survey of 502 citizens living near 19 biogas plants was conducted as the basis for using structural equation modeling to measure the effects of perceived benefits, perceived costs, trust towards the plant operator, perceived smell, information received and participation options on citizens’ acceptance of “their” biogas plant. Results show that local acceptance towards existing biogas power plants is relatively high in Switzerland. Perceived benefits and costs as well as trust towards the plant operator are highly correlated and have a significant effect on local acceptance. While smell perception and information received had a significant effect on local acceptance as well, no such effect was found for participation options. Reasons for the non-impact of participation options on local acceptance are discussed, and pathways for future research are presented.     

太阳能相变蓄热系统浅析

简要介绍了太阳能蓄热技术的发展背景,阐述了太阳能与相变蓄能的特点,说明了太阳能相变蓄热系统的工作原理,并对系统的特点进行了分析,归纳了系统在应用中面临的问题以及进一步的发展方向。     

Life Cycle Energy Assessment of biohydrogen production via biogas steam reforming: Case study of biogas plant on a farm in Serbia

The aim of this paper is to demonstrate and to quantify energy flows in a life cycle of biogas to biohydrogen production, starting from feedstock materials via anaerobic digestion, biogas upgrading, biohydrogen production, to the end of biogas system (application of digestate as fertilizer in agriculture). The performance of the biogas plant of Mirotin dairy farm in Serbia has been assessed. According to Life Cycle Energy Assessment approach, results obtained in this study have shown that biohydrogen production via biogas steam reforming has negative energy balance (with ?16,837 GJ). It has also been demonstrated that this process is energy unsustainable in an environmental context. In future analysis it would be necessary to consider the other aspects of sustainability, e.g. the economical and social factors in order to estimate the overall sustainability of the biogas utilization pathways, especially having in mind that the technology of converting biogas to hydrogen is still in the development phase.     

生物质能的气化能转化及其作用

世界上生物质资源十分丰富,对其进行现代化的利用,有利于减少环境污染、促进能源结构调整,因此引起国际上普遍重视。生物质能可用两种方法转化气化能,微生物法转化生物质能可获得高热值的沼气,而热解法则具有速度快、适应性广的特点。当前两种方法都有广泛的应用,代表着生物质能应用的主流。     

Trace compounds of biogas from different biogas production plants

Biogas composition and variation in three different biogas production plants were studied to provide information pertaining to its potential use as biofuel. Methane, carbon dioxide, oxygen, nitrogen, volatile organic compounds (VOCs) and sulphur compounds were measured in samples of biogases from a landfill, sewage treatment plant sludge digester and farm biogas plant. Methane content ranged from 48% to 65%, carbon dioxide from 36% to 41% and nitrogen from <1% to 17%. Oxygen content in all three gases was <1%. The highest methane content occurred in the gas from the sewage digester while the lowest methane and highest nitrogen contents were found in the landfill gas during winter. The amount of total volatile organic compounds (TVOCs) varied from 5 to 268 mg m⁻³, and was lowest in the biogas from the farm biogas plant. Hydrogen sulphide and other sulphur compounds occurred in landfill gas and farm biogas and in smaller amounts in the sewage digester gas. Organic silicon compounds were also found in the landfill and sewage digester gases. To conclude, the biogases in the different production plants varied, especially in trace compound content. This should be taken into account when planning uses for biogas.     

Trace element requirements of agricultural biogas digesters during biological conversion of renewable biomass to methane

The availability of trace metals as micro-nutrients plays a very significant role on the performance and stability of agricultural biogas digesters, which are operated with energy crops, animal excreta, crop residues, organic fraction of municipal solid wastes or any other type of organic waste. The unavailability of these elements in biogas digesters is probably the first reason of poor process efficiency without any other obvious reason, despite proper management and control of other operational and environmental parameters. However, trace metal requirements of biogas digesters operated with solid biomass are not often reported in literature. Therefore, the aim of this article is to review the previous and current literature about the trace metal requirements of anaerobic biogas digesters operated with solid organic substrates for production of methane.     

太阳能热水系统的节能效益分析

以呼和浩特北方宾馆为例，计算分析了该系统相关的节能效益指标，并列出了其他一些工程实例的节能效益指标，初步分析研究了太阳能热水工程的节能效益。给出了太阳能热水系统的增投资的回收期算法。     

聚焦太阳能建筑外观的能源论坛

第七届太阳能建筑外观能源论坛于2012年12月6～7日在意大利布列瑟农举行。本届论坛旨在为建筑师、工程师、科学家、能源管理者和光伏设备制造商创造对话机会,提供一个国际交流的平台,研究讨论出一种多学科的综合规划办法,在改善居住者舒适性和身心健康的同时减少能源消耗。能源论坛总经理Mr.Andreas Karweger在开幕式上致辞。     

Impacts of biogas plant performance factors on total substrate costs

Substrate costs in biogas production can be reduced by up to 30% by adjusting logistic capacity to chopper performance. The aim of this article is to analyze the impacts of biogas plant performance factors on total substrate costs for two common biogas plant sizes in Austria. A nonlinear optimization model is built to analyze the impact of alternative substrates, machinery chains, and field distances on total substrate costs of a 250 kW_el and a 500 kW_el biogas plant. The model minimizes total substrate costs subject to land which is available in different distant land circles around the plant. It optimizes machinery chains from planting to silo storage and considers nitrogen balances at field scales. Scenarios are constructed to investigate the impact of crop rotational constraints and alternative land availabilities on total substrate costs. Model results indicate that maize silage provides the least substrate costs. However, avoiding monoculture maize cropping can increase these costs by up to 29%. We also show that least biogas productions costs are not covered by current electricity prices in Austria.