Experimental investigation of air-source heat pump for cold regionsPompe à chaleur air-air utilisée dans les régions froides : étude expérimentale
Introduction
With the development of the national economy and improvement in people's living standards, the ASHP has been widely used for heating in central and south China in the winter since the 1990s [1], [2]. In these regions the outdoor temperature is comparatively high in the winter, thus the conventional ASHP available on the market can satisfy the heating requirement for these regions quite well. However, all theory and practice have shown that the conventional ASHP has a shortcoming that prevents its application in north China: the heating capacity of the ASHP decreases sharply as the outdoor temperature falls, thus in extremely cold climates—where the most heat is needed—heat pumps are least able to supply enough heat. Another drawback is that this kind of ASHP does not work reliably if the ambient temperature is below −5 °C because the COP of the ASHP becomes smaller under this circumstance than in the warmer outside temperature, and the discharge temperature of the refrigerant will continue to increase, leading to the destruction of the compressor unless the ASHP is stopped.
For a very long time the coal-fired boilers and coal-fired domestic stoves were the primary heating sources for people of north China. Unfortunately, with the development of society, the cities in these regions have grown, and these conventional heating methods do not meet the requirements of sustainable development since they contribute to severe environmental problems such as atmospheric pollution. Great efforts have been made to control the air-pollution level in these regions and seek sustainable alternative heating methods. Of all the available methods, the ASHP would perhaps be the most convenient if something could be done to solve the difficulties mentioned above. According to weather data [3], the ASHP is a feasible solution only if it can run smoothly under ambient temperatures as low as −15 °C; the conventional ASHP available on the market does not meet this criteria.
Much research has been conducted to enable the ASHP to run smoothly during winter in cold areas. In Japan, a new kind of ASHP was developed with a kerosene-fired burner either placed in the indoor unit or under the evaporator to improve the performance of the ASHP in low ambient temperature [4]. The packaged ASHP using a scroll compressor which varies the rotary speed according to the heating load and has liquid-injected inlets was proven to work smoothly even under low ambient temperatures of −10 to −20 °C [5], [6]. Since 1999 a SIG was instituted at Tsinghua University to work on technologies enabling the ASHP to work reliably and efficiently in north China [7]. Studies revealed that the heating capacity under low ambient temperatures can be greatly improved through the employment of a scroll compressor with supplementary inlets. A prototype ASHP was developed according to this conception, the dynamic performance characteristics were tested and the outcomes showed that this new kind of ASHP works very well in ambient temperatures as low as −15 °C. In addition, the efficiency of the improved ASHP under all circumstances was addressed and the improved system's increased efficiency can save energy. In this way the application scope of ASHP was enlarged and a simple and feasible alternative heating service was developed for heat in north China, which can also partly solve the severer and severer atmosphere pollution problem.
Section snippets
Analysis of the improved ASHP system
The principle of the improved ASHP is shown in Fig. 1 and the specifications of the prototype ASHP are listed in Table 1. The superheated refrigerant is discharged by a scroll compressor as a hot, dense vapor with high pressure (state 3) flows through a heat exchanger, the condenser, which transfers heat from the refrigerant to the water and becomes a saturated or sub-cooled liquid (state 4). Then the refrigerant as a higher pressure, cooled liquid from the condenser, is divided into two parts:
Experimental apparatus
Fig. 3 shows the test system for the dynamic performance of the prototype ASHP and the locations of the relevant sensors. In this system, the glycol solutions were used as the cooling medium for the evaporator which had an adjustable electric heater to keep the solution temperature stable. The condenser coolant was water, and the temperature was controlled by the air flow rate through the fan coil unit. Valves in the pipes allowed easy control of each of the operating parameters such as the
Heating capacity
The variations of heating capacity (Qh) with the evaporating temperature (te) are shown in Fig. 4 when the condensing temperatures (tc) were 45 and 48 °C respectively. The heating capacity decreased linearly with the decrease of the evaporating temperature, but rate of decrease was much slower than that of the conventional ASHP system. The heating capacity of the improved ASHP was approximately 5.5 kW when the condensing temperature was 45 °C and the evaporating temperature was −25 °C, and this
Conclusions
An improved ASHP was developed for relatively cold regions. The dynamic performances of the prototype ASHP were tested in a laboratory test that could control all the parameters. Results show the following:
- 1.
When the ambient temperature was near −15 °C, the prototype ASHP ran smoothly for a long time with enough heat to satisfy the heating requirements in cold regions.
- 2.
The heating capacity and power input of the ASHP will increase if the supplementary circuit is in use in low ambient temperatures,
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