تجزیه و تحلیل عملکرد سیستم پمپ حرارتی چند منظوره در حالت خنک کننده

A multi-functional heat pump system is proposed to effectively utilize waste heat and heat capacity in gray water for heating or cooling of residential buildings. Heat is also reclaimed from a plate heat exchanger installed at the discharge outlet of the compressor to provide sufficient hot water for residential use. To study the performance of this innovative system, laboratory testing has been performed with a prototype consisting of an outdoor heat pump, an indoor air handler, a gray water tank and a hot-water tank. This system is set in two environmental chambers that they mimic the outdoor and indoor environments, respectively. In this paper, the investigation of the proposed system is focused on the performance in cooling mode. The multi-functional heat pump system has been run under (i) space cooling mode and (ii) space cooling plus hot-water supply mode, with the same temperature conditions. The system performances in these two modes are compared and analyzed. The system is designed to allow four combinations of heat sinks with a water sink condenser and an air sink condenser. The four combinations are (1) air sink only, (2) water sink only, (3) air sink and water sink in parallel and (4) air sink and water sink in series, at the refrigerant cycle. Performance of the four combinations of heat sinks is experimentally investigated at a typical indoor air temperature of 26.7 °C and various outdoor air temperatures at 29.4 °C, 35 °C, and 40.5 °C. The results show that the heat sink combinations influence the cooling capacity and coefficient of performance (COP) of the system. The system performance and the optimal heat sink combination depend on the outdoor temperature. The impacts of outdoor temperature and gray water temperature on the performance of the system are discussed. The dynamic performance of the system for heating hot water from 30 °C to 48.9 °C is also studied. The proposed system has been shown providing significant energy savings in space cooling and hot-water supply. Moreover, the optimal source combination is critical in pursuing the maximum energy savings.

Water heating accounts for an average 18% of all residential energy use in the United States, which makes it to be the second largest use of energy in residential buildings [1]. In some states (e.g., California), the portion of energy use can reach as high as 25% of the total energy consumption. In a conventional building system, water heating usually is done by electricity or gas unit. However, this method consumes much energy compared with using heat pump systems. Therefore, heat pumps are becoming more popular for heating and cooling applications in residential buildings for energy saving. Ground-source and air-source heat pumps, as well as combining solar energy and geothermal heat pump, were proposed by many researchers [2]. Since the 1950s, research has been performed on heat pump water heaters [3] for energy saving. The potential energy sources (for instance, air and water) have been considered. Ito and Miura [4] have investigated the mechanisms of heat pumps for hot-water supply using combined air and water sources. The system can switch to either one or both heat sources. Direct expansion solar-assisted heat pump system that combined solar and air heat sources has been proposed to generate hot water [5], [6], [7], [8], [9] and [10]. Arif et al. [11] and [12] have been investigated exegetic modeling and performance evaluation of a solar-assisted domestic hot-water tank integrated geothermal heat pump systems for residential buildings has been performed. However, their research works are focused on saving energy in supplying hot water, and did not consider the potential energy saving with integration of air conditioning and hot-water supply systems. Heat pump water heaters for service water heating have hot-water production rate only 40–100% of that of the electric heating devices and 30–50% of that of the gas heating devices [13]. To provide quick recovery with this type of water heater, a household must have a large heat pump, an unusually large storage tank, and an electric backup heater. However, this electric backup heater will increase peak electrical demand and reduces overall energy efficiency [13]. Water heating is just a part of total energy consumption in buildings. In fact, the space heating and cooling consume a significant amount of energy. To further improve the energy efficiency of heat pumps in various applications, numerous researchers have investigated multi-functional heat pump system that not only provides hot water but also space heating and cooling. In residential buildings, the load of hot water can be satisfied by the multi-functional heat pump systems, meanwhile space cooling and heating can be provided. Ni et al. [14] investigated this type of system numerically, and showed the mean of daily hot-water load at a typical residential house in New York is about 33.6 MJ. The study is based on a calculation using the methods provided by Building America Research Benchmark [15]. Considering the usage profile [15] and [16], the mean hourly load of hot water is about 1.4 MJ. Through the numerical simulation, Ni et al. [14] concluded that the total source energy savings have a range of 17–57.9% among 15 cities in different climate zones in the U.S. using a combined heat pump system for hot-water heating and space heating and cooling. Reclamation of heat from the gray water was considered in their system. Hot-water heating has the most significant energy savings with over 60% reduction. Ji et al. [13] and [17] developed a prototype system and simulation program for an integrated domestic air-conditioner and water heater. This prototype only utilized the air source for heat source/sink. Kara et al. [18] and [19] applied the direct expansion solar-assisted heat pump for space heating, space cooling and hot-water supply. Ozgener and Hepbasli [11], [20], [21] and [22] developed a multi-function heat pump system by utilizing solar energy and geothermal heat. However, those previous research works did not investigate the impaction of combinations of heat sources/sinks on system performance. Ni et al. [14] have discussed the performance of a prototype of multi-function heat pump system with utilizing air and waste-water heat sinks. They have given the numerical study of the heat pump system and presented the system operation strategy and energy savings. Liu et al. [23] have experimentally studied the performance of the proposed system in heating mode. With a support from the U.S. Environmental Protection Agency, Hu [24] has developed a gray water treating system consisted of a simple screen, a bio-filter filled with shredded tire chips and a membrane bioreactor, which was intended to be applied to the proposed combined heat pump system with gray water source. In the proposed system, there are four types of combinations of air sink and waste-water sink for space cooling and hot-water heating, consisting of “air sink only,” “water sink only,” “air and water sinks in parallel” and “air and water sinks in series.” The “air sink only” solely uses the air-to-refrigerant heat exchanger in outdoor chamber for heat rejection, while the “water sink only” only uses the water-to-refrigerant heat exchanger located in the gray water tank. The “air and water sinks in parallel” represents the air-to-refrigerant and water-to-refrigerant heat exchangers, mentioned above, are configured in parallel. The “air and water sinks in series” represents the heat exchangers are configured in series. Five basic functions can be realized by this prototype system, which are space heating, space heating with hot-water supply, space cooling, space cooling with hot-water supply and hot-water supply only. In this paper, the system performance in different functions with different types of heat sinks combinations will be discussed. Nowadays, heat pumps become much widely used in residential buildings, especially using air-source heat pumps. The prototype system was modified from an existing air-source heat pump system. This paper will show the possibility and performance of modifying an existing air-source heat pump to a multi-function heat pump system. A significant energy saving on retrofitting existing air-source heat pumps with the multi-functional heat pump systems will be expected.

In this study, the multi-functional heat pump system shows superior performance in cooling mode compared with the conventional air-source heat pump system. The system with air and water heat sinks in series has the best performance in space cooling mode at outdoor air temperature of 35 °C and 29.4 °C and the second best performance at outdoor air temperature of 40.6 °C, compared with the system with other heat sink combinations. The performance of the innovative system with all types of heat sink combinations decreases as the outdoor temperature increases in space cooling mode and space cooling plus hot-water supply mode. Due to the discharge temperature of compressor distribution, space cooling plus hot-water supply mode is divided into two sub-modes, which are the heat exchanger located in outdoor environment is in force convection and natural convection, respectively. When outdoor air temperature is 35 °C, sub-mode I provides 2.66% and 12.77% more cooling capacity for space cooling only and total cooling capacity, respectively, compared with the cooling capacity of air sink only in the space cooling only mode. Also the COP for space cooling only and total space cooling of the system in sub-mode I are 3.68% and 13.97%, respectively, higher than the COP in space cooling mode. When the outdoor air temperature increases, sub-mode I has better COPs and cooling capacities than those of space cooling only mode. However, sub-mode I only has benefit at outdoor air temperature 40.6 °C and 35 °C. The sub-mode II is not restrained by the outdoor air temperature and can provide more capacity for supplying hot water than that of sub-mode I. In sub-modes I and II, the COP for space cooling and total COP decrease as the hot-water temperature increases. The capacity for supplying hot water decreases with the increase of hot-water temperature. The process of heating hot water from 30 °C to 48.9 °C usually takes less time for sub-mode II than that for sub-mode I. Gray water temperature has limited impacts on parallel heat sink combinations and has significant impacts on series heat sink combination and water sink only. The lower bound estimated performance of the heat pump system is better than the conventional system.