BES energy analysis of a combined heat & power plant utilizing livestock manure with its economic analysis

Transcription

1 Ref: 1064 BES energy analysis of a combined heat & power plant utilizing livestock manure with its economic analysis Se-jun Park, and In-bok Lee, Department of Rural Systems Engineering, Research Institute for Agriculture and Life Science, ollege of Agricultural and Life Sciences, Seoul National University, 599, Gwankno, Gwanku, Seoul, , Republic of Korea Abstract The combined heat and power plant is a high-efficient energy system that produces either electricity and valuable heat from a single fuel source. Recently, disposal of livestock manure is one of the serious problems of livestock industry, but this problems can be solved by utilizing the biomass fired cogeneration plant using livestock manure. There are some factor which can affect to efficiency of cogeneration plant using livestock manure like time-dependently changes of natural weather conditions. The complicated connection with each systems must be considered for predicting energy production. However, most conventional design method can t consider these things. Because it can lead to uncertain design capacity of cogeneration, a design method which consider various weather conditions. Hence, we developed the cogeneration plant model using livestock manure considering environmental conditions in real time, and we computed the generation of electrics as well as thermal energy including energy recovery. In this study, we developed the Building Energy Simulation (BES) model of the combined heat and power plant using bio-gas produced from livestock manure. To validate the integrated BES model of the cogeneration, several important raw data such as amounts of generated electricity and emission gas, temperatures of heat storage tank, emission gas, internally designed air temperature of livestock house, etc were measured at a commercial livestock farm. Then we computed the amount of greenhouse gas reduction, and then the economic analysis was conducted based on the computed results using RETScreen. As a result, the energy to maintain optimum temperature of anaerobic digester for producing bio-gas was estimated at GJ, and the recovered energy from waste heat recovery system was estimated at GJ per year. In addition, the cogeneration system could reduce greenhouse gases from to to 2 compared with conventional heating system. The economic analysis was conducted by considering the oil price at 2013 and the payback period was estimated to 4.5 years. In conclusion, the integrated BES and RETScreen model for evaluating the system design and its economic analysis can be very valuable tool for real designing criteria of combined heat and power plant using livestock manures and suggest proper scale of cogeneration plant of farm. Keywords: Building Energy Simulation, ombined heat and power plant, Economical analysis, Livestock manure 1. Introduction As fossil fuel which is a limited amount is being exhausted, the energy crisis have been constantly emphasized. In addition, the increasing use of fossil fuel lead to growing Proceedings International onference of Agricultural Engineering, Zurich,

2 greenhouse gas emissions. For reducing the greenhouse gas emissions, various the Framework onvention on limate hange had also been made an agreement at the 1990s. To cope with mentioned situation, many countries have striven for preparing solutions like reducing consumption of fossil fuels, alternative energy source which is abundant and do not emit a greenhouse gas. So they have tried to develop renewable energy such as wind power energy, geothermal energy, and solar energy etc. These renewable energies have many limitations such as weather conditions, geographical conditions and so on. The wind power energy needs constantly strong wind. The solar energy are nonetheless expensive to produce yield only limited energy output. However, a bio-energy is relatively free from mentioned limitations. Although bio-energy present very variable productivity, an anaerobic digestion method that produces biogas by organic waste tract biomass has been recognized as most valuable technology(krei, 2011). The anaerobic digestion has been regarded as a good method for stabilizing treatment of organic waste tract biomass (Lettinga, 2001). The method has been evaluated as eco-friendly recycling treatment, because an odor can significantly decreasing after anaerobic digestion process (Kim et al., 2012). The productivity of manurebased biomass is normally relatively lower that other energy sources, many studies have been conducted to improve its productivity and designing small & medium size biomass fired cogeneration plant which has small thermal loss than big size cogeneration plant. The livestock sludge is treated through liquefied fertilizer, marine exhaustion, discharge, and public treatment. A pollutant load of livestock manure is worse than loads of domestic sewage. Therefore, it is helpful for protection of the environment as well as producing biomass using livestock manure. The biomass fired cogeneration plant using livestock manure is composed with several production facilities and these facilities have a dynamic relations. However, there are many difficulties to consider their organic process and relationship. Moreover, most of the design method of the anaerobic digestor which generate biomass haven t considered variable weather condition while it use static weather condition. As a result, this method can cause excessive or under estimation on the design of cogeneration plant s capacity like generator, heat storage tank etc. So, our goal of this study was to develop the biomass fired cogeneration plant model using BES (Building Energy Simulation) that can consider variable weather data and system s organic process and relationship. In this study, TRNSYS which one of BES program, was used to design the biomass fired cogeneration plant using the different subroutines which simulate the individual energy components. The economic analysis was conducted about designed cogeneration model. Finally, this method of design cogeneration plant suggested about proper scale of cogeneration system according to scale of farm. 2. Materials and methods 2.1 Experimental livestock farm A Gwang-il farm was adopted to design and validation of simulation model. The Gwang-il farm is located in Anseoung-si, Gyeonggi-do, Korea and it was built in Decemver of This farm has a biomass fired cogeneration plant and has a 4,000-pigs in 13,223 m 2 scale pigsty. In the cogeneration system, two of 200 ton scale anaerobic digestor were operated and it produced biogas using 15 ton of livestock manure per day. The cogeneration system usually generates electricity all the time using two generator alternately. The electricity generated by biomass fired cogeneration plant is used in the farm and the recovered heat energy is firstly used to maintain optical condition of the anaerobic digestor, and it is secondly used to heating dormitory which is located in Gwang-il farm. 2.2 Building Energy Simulation (BES) modeling Building Energy Simulation (BES) is very powerful method for estimating heating & cooling loads of structure, because it can consider the time dependently changed weather conditions such as air temperature, humidity, wind direction, wind speed and solar radiation, etc. In Proceedings International onference of Agricultural Engineering, Zurich,

3 addition, BES has many modules and dynamic link between each modules and it enable to simulate an energy flow between each modules. So, BES is used by engineers and researchers around the world to validate new energy concepts, from simple domestic hot water systems to the design and simulation of buildings and their equipment. Especially, TRNSYS (ver.17, WI, USA) which is one of the BES commercial program is developed by Solar Energy Laboratory at Wisconsin-Madison University and it was commercialized for analysis of solar energy systems in Now, it has been upgraded until 17 version and is widely. TRNSYS program has various modules such as can simulate the behavior of transient systems by connecting each modules of system and using real-time date like variable weather date. In this study, the biomass fired cogeneration plant was designed as shown in Fig. 1 using TRNSYS. The biomass fired cogeneration plant was consisted of the generation system, the waste heat recovery system, the utilization of waste heat system, and so on. Table 1 explains the main modules of the systems used for BES programs The generation system using engine module The generation system was composed of generator module and input module of daily manure supply. In this research, it was applied that generator was supplied 20 m 3 of methane gas per 1 m 3 of livestock manure according to field data suggested by hoi et al.(2003). They indicated that mesophilic anaerobic digestor produces methane gas at 35 internal temperature, 7~8 ph. In this research, it was assumed that the anaerobic digestor maintain optimum condition of producing methane gas such as internal temperature, ph etc. As a results, the anaerobic digestor can produce 20 m 3 of methane gas per 1 m 3 of livestock manure The waste heat recovery system using heat exchanger module onsidering Gwang-il farm s system, the waste heat recovery system was designed to transfer the waste heat energy from hot exhaust gas to water of heat storage tank and then it can be used for heating anaerobic digestor and dormitory. The efficiency of heat exchange module was calculated by difference of temperature between exhaust gas and inflow water temperature of heat storage tank according to field data. The heat exchanger module calculate the total transfer rate(q T ) across heat exchanger by the following expressions. Q T = εq max Q max = minimum value of h and c (T hi T ci ) where, Q max is the maximum heat transfer rate across exchanger, h and c is the capacity rate of fluid on hot side and on cold side, T hi and T ci is the hot side and cold side inlet temperature, Q T is the total heat transfer rate across heat exchanger and ε is the ehat exchanger effectiveness. In this system, the heat exchanger is modeled in the shell and tube modes. This mode estimate the effectiveness(ε) by the following expressions. ε 1 = 2 ε = 1 ε min 1 ( max ) 1 ε 1 [ N 1 ε min 1 1 ( max ) 1 ε 1 ] [ N min max ] 1 + min + (1 + ( exp [ UA (1 + ( min) min ) ] max min ) ) max max 1 exp [ UA { (1 + ( min min ) ) ] max } Proceedings International onference of Agricultural Engineering, Zurich,

4 where, max and min is maximum and minimum capacity rate, UA is overall heat transfer coefficient of exchanger and ε is heat exchanger effectiveness. A schematic of the heat exchanger is shown in Fig The utilization of waste heat system The waste heat was firstly used to maintain optimum condition of anaerobic digestor for producing biogas, and then used to heat dormitory. The heating loads for maintain optimum condition of anaerobic digestor for producing biogas were calculated by the equations shown in below. Q Total = Q 1 + Q 2 Q 1 = W(T out T in ) Q 2 = UA(T a T d ) where, Q Total is the total heating loads, Q 1 is the heating velocity (kcal/hr), W is flow rate of sludge (kg/hr), is specific heat of sludge (kcal/kg ), T in is sludge inlet temperature ( ),T out is sludge outlet temperature ( ), Q 2 is thermal losses of anaerobic digestor (kcal/hr), U is heat transfer coefficient (kcal/m 2 hr K), A is area of a anaerobic digestor surface (m 2 ), T a is ambient temperature ( ) and T d is inside temperature of anaerobic digestor ( ). 2.3 RETScreen RETScreen (ver. 4, ANMET Energy Diversification Research Laboratory, anada) was developed for sustainable energy project by EDRL that was composed of anadian government and related industry, and it is the commercial program for economical evaluation of renewable energy. RETScreen has been widely used on economic analysis. For example, Jesse et al.(2006) studied geothermal heat pump system and Hamane et al.(2009) conducted hydrogen production system from wind power. In this study, the biomass fired cogeneration plant designed using BES was assessed to aspect of economic feasibility and environmental effect using RETScreen. The heat energy and electricity which were estimated by BES simulation were input data for RETScreen program. So, the economic feasibility was conducted about boiler system and biomass fired cogeneration plant. Two case were compared which one was best, and environmental effect assessment was also performed by calculating reduction of greenhouse gas emission. 3. Results and Discussions 3.1 Electricity performance of generation system The results of electricity performance using BES presented that generation system was affected by two factors. The first factor was supply of manure. Because the amount of methane gas produced by manure could decide operating hours of generator, the manure supply was important factor of electricity performance. Another factor was ambient temperature. The engine module in BES calculate electricity generation considering ambient temperature. The results presented that an amount of electricity generation in summer season was larger than winter season as shown in Table 2. This results showed that ambient temperature of generator was proportional to generating efficiency. 3.2 Heat recovery performance of waste heat recovery system The conditions of anaerobic digestor like temperature, ph seriously affected to production of biogas, therefore the recovery system of thermal energy was firstly adopted to maintain optimum condition of anaerobic digestor. The manure supply was main factor which affected Proceedings International onference of Agricultural Engineering, Zurich,

5 amounts of heat recovery from waste heat energy because it was fuel of generator. However the manure supply was relatively uniform in this study, it could not make a big impact on the variation of heat recovery performance. On the other hand, the heating load for maintaining optimum condition of the anaerobic digestor showed big variation because of the seasonal effects. It was caused by thermal losses of anaerbic digestor. The thermal losses of the anaerobic digestor were mainly affected ambient temperature, so it was large at winter season. The quantities of heat from the recovery system usually were much higher than heating loads for the anaerbic digestor except summer season. This results showed that the amount of heat energy which recovered by cogeneration system could not independently cover total required heat energy using in the farm so, this farm needed some auxiliary heating system as shown in Fig. 3 qualitatively and Table 3 quantitively. The maximum heating loads for maintaining the optimum condition of anaerobic digestor were 23.62kJ at 1 February when the lowest air. At that time, the heat recovery energy from cogeneration system was 8.95kJ and it could not satisfy the maximum heating loads. So, this result could be used for deciding the capacity of auxiliary heater. 3.3 RETScreen The environmental and economic analysis of the cogeneration plant were conducted using RETScreen. There were two cases according to fuel type of cogeneration. The first one was system using biogas and another was system using fossil fuel. The generation system using fossil fuel was estimated that it emitted 409.2tO 2 per year, while the cogeneration system fired biomass presented that it emitted 323.3tO 2 per year. These showed that the cogeneration system which fired biomass was better plant than power plant fired fossil fuel. The economic analysis of cogeneration system which fired biomass showed that the internal rate of return (IRR) was calculated at 25% and equity payback period was calculated at 4.5 year. 4. onclusions In this study, the simulation model of biomass fired cogeneration plant was developed using BES and economic analysis of this model was conducted. The productivity of electricity was mainly affected by amount of biogas which was fuel of generator, and was a little affected by ambient temperature. However, the recovery heat energy and the heat loads for maintain optimum condition of anaerobic digestor were mainly affected by ambient temperature. There are big difference of each loads according to season. As the heating loads such as mentioned two loads are important factor in biomass fired cogeneration plant, they have to consider for modeling of cogeneration system. The analysis results using RETScreen presented that cogeneration plant can reduce much O 2 emissions and its payback period is enough short respectably. 5. References Dong, S.., J. Z. Wen and J. Nathwani Simulation of cogeneration within the concept of smart energy networks. Energy onversion and Management. 75: Hamane, L. A., M. Belhamel, B. Benyoucel and M. Hamane Feasibility study of hydrogen production from wind power in the region of Ghardaia. International Journal of Hydrogen Energy. 34(11): Jesse D. M., M. R. Andrew and V. K. ornelis Wind integration into various generation mixtures. Renewable Energy. 34(3): Kim, J. A., Y. M. Yoon, K. H. Jeong and. H. Kim Effects of Supplementation of Mixed Proceedings International onference of Agricultural Engineering, Zurich,

6 Methanogens and Rumen elluxlolytic Bacteria on Biochemical Methane Potential with Pig Slurry. Korean Journal of Soil Science and Fertilizer. 45(6): KREI Prospect of production and utilization of energy in the rural sector and strategies for introducing clean energy farming system. KREI, Seoul, Korea. Lee,. Y haracteristics of Methane Production from Piggery Manure Using Anaerobic Digestion. Korea Organic Recycling Association. 15(3): Lettinga, G Digestion and degradation, air for life. Water Sci. Technol. 44: TRNSYS Documentation Volume 1 Getting Started Figure 1 The biomass fired cogeneration plant modeled using TRNSYS Figure 2 Schematic of heat exchanger in waste heat recovery system (That neither the cold nor the hot side fluids are mixed) Proceedings International onference of Agricultural Engineering, Zurich,

7 Figure 3 BES simulation results of Heat recovery performance (red line is ambient temperature, yellow line is recovery heat energy from cogeneration and blue line is heating loads for maintain to optimum condition of aerobic digestor for producing bio-gas) Table 1 Main modules of cogeneration system for BES program Module Name Description Marathon Engine Generator Pipe Water Pump Storage Tank Exhaust Recouper ator HXer2 This component models an engine generator. A device used to generate electricity by burning fuel in an internal combustion engine. The model relies on an external data file which contains efficiency, air flow rate and heat transfer data as a function of the intake temperature and the part load ratio. This component models the thermal behavior of fluid flow in a pipe or duct using variable size segments of fluid. This pump model computes a mass flow rate using a variable control function. The thermal performance of a fluid-filled sensible energy storage tank. Fluid entering the hot side of the tank is added to the tank node below the first auxiliary heater. Fluid entering the cold side of the tank enters the bottom node This sensible heat exchanger is a shell and tube type device Given the hot and cold side inlet temperatures and flow rates, the effectiveness is calculated for a given fixed value of the overall heat transfer coefficient. This sensible heat exchanger is modeled as a constant effectiveness device which is independent of the system configuration. Table 2 Seasonal electricity generation based on amount of fuel (Unit: GW) Winter season (Jan., Feb., Dec.) Summer season (Jun., Jul., Aug.) Total period Electricity generation Table 3 Analysis of monthly heat recovery energy and heating loads for optimum condition of anaerobic digestor for producing bio-gas (Unit: GJ) Month Amount of heat recovery from waste heat energy (a) Heating loads for maintain optimum condition of anaerobic digestor (b) Difference (a) with (b) Ambient temperature ( ) Proceedings International onference of Agricultural Engineering, Zurich,

8 Sum Proceedings International onference of Agricultural Engineering, Zurich,