零排放的新除湿空调
Energy Saving Effect of Novel Desiccant Air Conditioner
for Zero Emission House
Shun Hirano 1, Chanyong Park 1, Tsukasa Hori 1, Yoshinori Hisazumi 1,
Tsutomu Wakabayashi 2, Akira Kishimoto 2
1
Exergy Design Joint Research Laboratory, Osaka University, Osaka, Japan 2
Energy Technology Research Laboratories, Osaka Gas CO., Ltd., Osaka, Japan
Abstract
The model of zero emission house [ZEH] made full use of leading-edge energy saving and new energy technology of Japan was exhibited at G8 Hokkaido Touyako Summit held on July 2008. Introduction in Japan is set as the goal of starting popularization of ZEH from 2015, change all newly-built houses to ZEH by 2030 and change all houses including existing house to ZEH by 2050. We have developed new style desiccant air conditioner which can supply dry cool air in summer and warm humidified air in winter by using heated water of solar heat or CGS. In this paper, we publish the energy saving efficiency of the incorporation of this desiccant air conditioner into ZEH and case example of exergy analysis compared with heat pump systems in summer extremely hot day.
Keywords:
ZEH, CGS, Heat Pump, Desiccant Central Air Conditioner, Exergy
1 INTRODUCTION
Due to the impact of the Great East Japan Earthquake on March 11 and the accompanying accident in the Fukushima Daiichi Nuclear Plant, there is an increasing demand for energy saving. At the same time, zero emission houses (ZEHs) for consumer use are attracting attention as a measure against global warming, and their demonstration is being promoted in various parts of Japan [1]. ZEH is a housing design aimed at avoiding CO 2 emissions by using photovoltaic power generation and other methods. Osaka Gas is conducting demonstration tests of a smart energy house (SEH) that utilizes photovoltaic power generation, fuel cells and storage batteries. The air conditioning in this SEH combines a commercial air conditioner with a desiccant unit. With focus on solar water heaters and gas engine cogeneration units of improving efficiency for domestic use, we are developing ventilation and central air conditioning for detached houses using the hot water generated by the units [2]. This air conditioning system ably combines dehumidification and regeneration capacity of desiccant and cooling capacity of water spray with air passage switching function to attain dehumidification cooling and humidification heating functions similar to those of air conditioners. The effect of this system, which is currently under development, on reduction of power consumption in households in summer was assessed with the process simulator VMGSim using assumed outdoor temperature and humidity values, preset indoor temperature and ventilation air flow as the parameters. In addition, the performance of the devices constituting this system was evaluated to identify the devices to be improved to reduce exergy loss, and its anticipated primary energy saving
effect was compared with that of a conventional heat pump air conditioner and hot water energy supply system using grid power.
2 IMPROVEMENT OF HEAT & POWER SYSYTEM FOR DOMESTIC USE
As a commercial product to compete with the natural refrigerant heat pump hot water heater called “Eco Cute ” released in 2001, “Eco Will,” a gas engine power generation system for domestic use, was put on sale in 2003. Table 1 shows the changes in its specifications. While the temperature of the stored hot water is 75 o C, the maximum temperature of the supply water is 80 o C due to an upper limit to the temperature of the hot water for cooling the main unit of the gas engine; also, the power generation efficiency on a lower heating value basis has been increased to 26.3% from 20% at the time of the release. In consideration of the trend for improvement in the power generation efficiency of the tens of kW class of gas engines, it is expected that power generation efficiency of gas engines for domestic use will exceed 30% in the future.
Table 1: Changes in the specifications of gas engine for
domestic use.
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OI 10.1007/978-94-007-3010-6_27, ? Springer Science+Business Media Dordrecht 2012
D ,Design for Innovative Value Towards a Sustainable Society M. Matsumoto et al. (eds.),
The fuel cells for domestic use such as SOFC, which currently attract attention as high-efficiency power generation units, are being developed with a target of commercializing units with power output of 700W and hot water output of 620W (with power generation efficiency of 45% and waste heat utilization efficiency of 40%). While we have proposed a system that can also use the high-temperature waste gas from solid-oxide fuel cells as air for regenerating desiccant, this paper assesses the energy saving effect of the system that can use waste heat from the gas engine already available in the market as part of the heat source for regeneration of the desiccant.
3 CENTRAL AIR CONDITIONING SYSTEM USING HOT WATER
Figure 1 illustrates the central air conditioning system using the hot water generated by a gas engine cogeneration unit for domestic use and a solar heater. The air conditioner is installed near a gas engine cogeneration unit with power output of 1kW, and the dehumidified cool air processed by the unit is supplied through air ducts in summer to various rooms including living room, dining room, bed room, study room and children’s room. The cooling heat load for the central air conditioning with outdoor temperature of 35 o C and indoor temperature of 27o C is assumed to be up to 4kW in a detached house with a floor area of 120 m2in the Kansai area, based on the coefficient of heat loss specified in the energy-saving standard for houses [3].
Figure 2 shows the equipment configuration of the whole system, and Figure 3 presents a psychrometric chart on the dehumidifying and cooling operation. The air conditioning system is outlined below. Hot water of approximately 80 o C heated with solar heat and waste heat from the gas engine is used as heat source for regeneration of the desiccant rotor. During mid-day hours, when the cooling heat load is large, with the aim of increasing the cooling heat load, indoor air in the amount equivalent to the air taken in from the outside is introduced to the desiccant rotor and dehumidified to lower the absolute humidity to around 8g/kgDA. The process air, heated to about 48o C with the heat generated as a result of the dehumidification, is cooled with indoor air and supersaturated outdoor air. The cooling heat load is set to be 4kW. Then, in consideration of the moisture of 330g/h generated from human bodies, etc., water is sprayed to the process air to reduce the absolute humidity to below that of the indoor air. After spraying, the dehumidified cool air of around 22 o C is supplied through air ducts to the rooms. The psychrometric chart indicates that the temperature of the regeneration heat source can be lowered with the application of same amounts of outdoor and indoor air to the desiccant rotor. The ventilation air flow to generate cooling heat load of 4kW is 390 m3/h, and the cooling air supply is about twice this amount.
The devices of the proposed system are configured such as to allow consideration of the cases where solar heat or waste heat from the gas engine cannot be used and where the regeneration air can be heated with city gas burner to increase the cooling heat load. Solar heat received during the daytime is stored in the hot water storage tank and can be used at night as a heat source for regeneration of the desiccant.
Figure 1: Image of the central air conditioning system.
Figure 2: Equipment configuration of the whole system.
Figure 3: Psychrometric chart during dehumidifying and
cooling operation.
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Figures 4 and5 respectively show temperature-heat load characteristics of the process air cooler (to cool the process air with indoor air and water-sprayed supersaturated air) and the regeneration air heater (to heat indoor air with hot water) that can address a cooling heat load of 4kW. The exchange heat quantity in Figure 4 less the heat input of the blower is the cooling heat load for the desiccant air conditioning system. Since the heat load of the hot water to heat the regeneration air in Figure 5 exceeds the waste heat collected from the gas engine with rated power output of 1kW, the shortfall will be met with the solar heat stored in the hot water storage tank. Relatively high thermal performance is assumed based on the premise that the minimum approach temperature of the heat exchangers is around 5 o C.
Figure 4: Temperature-heat load characteristics of process
air cooler.
Figure 5: Temperature-heat load characteristics of
regeneration air heater.
Figures6 and7 show the Sankey diagrams of an energy supply system with dehumidification cooling and hot water supply functions using waste heat from gas engine and city gas burner on enthalpy and exergy bases, respectively. Based on the exergy assessment of this system, the largest losses in the gas engine are generated in connection with combustion in the cylinder and cooling in the cylinder jacket, while the largest losses in the dehumidification cooling system stem from use of the city gas burner, efficiency of the blower, and heat exchange of desiccant rotor, hot water heater, etc. In the desiccant unit in Figure 6, the latent heat generated by spraying water to the dehumidified supply air is added to the cooling heat load mentioned above. Quantitative assessment is described in the examination of the factor analysis.
Figure 6: Sankey diagram based on an enthalpy basis.
Figure 7: Sankey diagram based on an exergy basis.
4 AIR CONDITIONING AND HOT WATER SUPPLY SYSTEM USING HEAT PUMP
The "top runner system" enforced by the Energy Saving Act in 1999 mandates the display of energy consumption efficiency, etc. on products, and currently designates 23 kinds of equipment including air conditioners and hot water supply systems. The efficiency has direct impact on the sales of air conditioners in particular, and competition for improving their efficiency has intensified.
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refrigerant heat pump water heater, which significantly saves energy in comparison with traditional electric water heaters, is included in the 10 items of the JSME Technology Roadmap. According to the literature, the energy consumption coefficient (coefficient of performance, COP), which is determined by dividing the quantity of heat production by energy input, is anticipated to increase from 3.5 at the time of the release in 2001 to 6.0 in 2030. These air conditioners and heat pump water heaters are currently obligated to display their efficiency in terms of APF (annual performance factor).
Based on the values shown in the catalogs of these products, we have developed a process simulation model to evaluate power consumption for dehumidification cooling and water heating, using outdoor temperature and humidity, preset indoor temperature, and ventilation air flow as well as the temperature of input water and heating temperature of water heaters as the parameters. Figure 8 illustrates the model. The air conditioner is a model with regenerative dehumidification capacity. R410A and CO 2 are assumed to be the refrigerants for the air conditioner and heat pump water heater, respectively. The power consumption can be calculated based on the outdoor moisture and heat taken in by ventilation and heat leakage from outside of the building in addition to the operation specifications of the desiccant mentioned above. The operation time and power consumption of the heat pump water heater to supply the same amount of hot water supplied by the gas engine cogeneration system mentioned earlier can be calculated in consideration of heat radiation
during the storage of the hot water.
Figure 8: Equipment configuration.
Figure 9: Sankey diagram based on an enthalpy basis.
Figures 9 and 10 show the Sankey Diagrams of this system based on enthalpy and exergy respectively. Exergy in the heat pump is mainly lost from the compressor, valve, evaporator, and condenser. As the temperature of the stored hot water in the hot water supply system is usually as high as 65 o C or more, the mixing process of hot water and tap water during use causes large losses.
Figure 10: Sankey diagram based on an exergy basis.
5 QUANTIFICATION AND COMPARISON OF LOSS IN THE ENERGY SUPPLY SYSTEM FOR DOMESTIC USE
Based on data from Japan Meteorological Agency, typical changes in the temperature and humidity for an extremely hot day in Osaka and Kyoto this year are simulated and presented in Figure 11. As the sun goes up and the outdoor temperature rises, relative humidity declines. On the other hand, absolute humidity is constant at approximately 17g/kgDA throughout the day unless the weather changes, and discomfort index is over 77 even at night, at which
half the people feel uncomfortable.
Figure 11: Outdoor conditions of an extremely hot day.
The power consumption and hot water requirement under the outdoor conditions simulated above were determined with indoor temperature of 27 o C, absolute humidity of 12g/kgDA and ventilation air flow of 390 m 3/h.
With the outdoor temperature of 30 o C or more, the process air flow was assumed to be twice as much as ventilation air flow to increase the cooling capacity. For the purpose of calculating the operation time of the cogeneration system and heat pump hot water
heater, the
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daily hot water requirement was set to be 6kWh. To simplify the comparison of different systems, the energy saving effect of photovoltaic power generation was excluded. Although it was necessary to consider the axle efficiency of the rotary machine, inverter efficiency, and motor efficiency to calculate the power of the rotary machine, only the information on power consumption in the catalog was available, and adiabatic efficiency was therefore used to calculate the power consumption of the rotary machine in the process simulator.
5.1 GAS ENGINE SYSTEM FOR DOMESTIC USE For the examination of this system, two cases on power generation efficiency of the stoichiometric gas engine for domestic use were selected, namely 2
6.3% (the current value) and 30% (a value expected in the future), and the temperature of the hot water supply was set to be constant at 80 o C. The waste heat utilization efficiency in the two cases was respectively set to be 63.7% and 58% in anticipation of some heat loss due to radiation, and the gas engine was assessed on an exergy basis temperature with the outdoor temperature of 35 o C. The assessment results with the input fuel to be 100 are shown in Table 2.The overall thermal efficiency is respectively 92% and 90% on an enthalpy basis and 30.1% and 32.9% on an exergy basis. It is obvious that improvement in power generation efficiency results in reduction in loss from the radiator. The energy requirement on a lower heating value basis in case of operating the gas engine from 7 a.m. to 11 p.m. is tabulated. Table 3 shows the exergy analysis of the desiccant air conditioner under different outdoor conditions as well as its power consumption according to the specifications and operation time. The types 35-A, 31-B, 29-C, 27-D, and 35-E involve different outdoor temperatures and supply air flows. The Arabic numerals represent the outdoor temperature, and the supply air flow is approximately 750 m3/h in Types A, B and E, and 380 m3/h, or about half of the former, in Types C and D. Types A and B are operation with high cooling capacity additionally using solar heat; Type C is operation with intermediate cooling capacity using waste heat from gas engine; and Type D is dehumidification operation using the heat of the stored hot water. In Type E, city gas is combusted to increase the cooling capacity. The results of the exergy analysis of the desiccant air conditioner shown in Figure 7 and the assessment of loss in this table demonstrate that the use of city gas for heating the regeneration air generates very large amount of combustion loss. In the calculation on the process, the dehumidification efficiency of the desiccant rotor was set to be 85% or less, and the head and efficiency of the blowers to be 350 Pa and 35%, respectively. For the heat transfer property of the heat exchangers, the minimum approach temperature with the maximum load was set to be 5 o C. In the exergy evaluation, the outdoor temperature was different between the cases. When the evaluation temperature dropped, the exergy loss in the blower efficiency increased correspondingly. The power consumption of the blower in case of operation for 24 hours a day is shown in the lower right of the table.
Table 2: Evaluation of the gas engine.
Table 3: Evaluation of the desiccant air conditioner.
The amount of dehumidification by the desiccant air conditioner, which depends on the difference between indoor and outdoor absolute humidity, is 2.0 to 2.6 kg/h with ventilation air flow of 390 m3/h. The room cooling capacity is influenced by regeneration temperature and outdoor absolute humidity, and increases as the temperature of the supply air falls. Even when the regeneration air temperature is 72 o C, the supply air can be cooled to 20 o C or less with water spray if the outdoor absolute humidity drops.
The cooling capacity, including the latent heat of the water, exceeds 5kW in Types A and B.
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5.2 HEAT PUMP SYSTEM
Using the process simulator based on the values in catalogs prepared by manufacturers, the latest air conditioner was considered to have the minimum approach temperature of 3 o C, compressor with 75% efficiency and blower with 40% efficiency under its rated capacity. Table 4 shows the exergy evaluation results and power consumption of the air conditioner and heat pump hot water heater. As the outdoor temperature and dehumidification amount decrease, the power consumption of the compressor and blower falls. At the outdoor temperature of 35 o C, the COP of the air conditioner and heat pump hot water heater were 5.15 and 4.72 respectively, while their exergy values were 7.1% and 31.9% respectively. In the heat pump hot water heater, the effective exergy efficiency dropped to less than half due to mixing with supply water and heat radiation from the hot water storage tank.
5.3 ASSESSMENT OF ENERGY SAVING EFFECT
AND SURPLUS POWER
The replacement of the existing air conditioner with the
proposed desiccant air conditioner is expected to produce
energy saving effect of 1,574W (net output of gas engine: 925W, power consumption of desiccant: 384W, and power
consumption of air conditioner: 1,033W). Then, efficiency in the generation of surplus power was evaluated in the two cases of the grid power conversion
efficiency of 40% (the current value) and 55% (the
efficiency of the latest gas turbine combined cycle) on a
lower heating value basis. The daily power consumption
of the air conditioner and hot water supply was divided by
the grid power conversion efficiency mentioned above to determine the primary energy requirement.
The primary energy corresponding to the surplus power of the gas engine is the daily energy consumption of the gas engine less the above energy requirement. Accordingly, the efficiency in the generation of surplus power can be calculated by subtracting power requirement of the desiccant air conditioner from net power consumption of the gas engine and dividing the difference by the primary energy corresponding to the surplus power. With blower efficiency of 35% and 55%, power surplus is respectively 6,693Wh and 9,470Wh. The values determined with the calculation mentioned above are shown in Table 5.
Table 5: Efficiency in the generation of surplus power.
6. CONCLUSION The exergy analysis has identified the components of the energy supply systems that need to be improved in the
future. Especially, the energy supply system using the existing gas engine cogeneration unit for domestic use is more beneficial than the combination of existing power system efficiency and Eco Cute, but has no advantage when compared with the latest combined cycle because of the exergy loss due to combustion and heat transfer in the heat source device, and it is therefore necessary to improve the cogeneration efficiency. Meanwhile, in the central air conditioning system using desiccant, which is currently under development, it is essential to improve the blower efficiency. Nevertheless, in light of the current situation that peak power consumption has to be reduced, the air conditioner that makes use of waste heat at the peak power time in summer is attractive even if it is based on gas engine for domestic use. In the future, the power saving effect of the energy supply system in winter and throughout the year, as well as the effect of replacement of
the heat source equipment with SOFC will be examined from the aspect of the exergy design of the devices.
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A., Enomoto, H., Ueda, K., Enthalpy and Exergy Analysis of Domestic Desiccant Air Conditioner with Cooling Dehumidification and Heating
Humidification Using Process Simulator, ICOPE Technical publication POWER 2011-55368. ?3?Sakamoto,Y., History and Future of Energy Conservation in Japanese Housing Sector, JSER, VOL.31 NO.3, 2010.5, 7-10, (in Japanese).
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