Shengchun Liu

The optimal charge of carbon dioxide in water–water heat pump systems with and without an internal heat exchanger

ABSTRACT An experimental study was conducted on the optimal charge of carbon dioxide in water–water heat pump systems (WWHPSs) with and without an internal heat exchanger (IHX). An appropriate carbon dioxide charge standard was obtained from the experimental results. All tests were conducted with the inlet and outlet water temperatures set to 12°C and 7°C in the evaporator, respectively and 20°C and 55°C in the gas cooler, respectively. During the tests, the evaporation temperature was 0°C. The coefficient of performance of cooling (COPc) and coefficient of performance of heating (COPh) increased by 8.3% and 0.7% in the WWHPS with an IHX compared with the WWHPS without an IHX. Under the test conditions, the optimal carbon dioxide charges with and without an IHX were found to be 2.0u2005kg and 2.25u2005kg, respectively. The test results show that the optimal carbon dioxide charge decreases by 11.1% when an IHX is included in the WWHPS.

Building integrated solar thermal (BIST) technologies and their applications : A review of structural design and architectural integration

Solar energy has enormous potential to meet the majority of present world energy demand by effective integration with local building components. One of the most promising technologies is building integrated solar thermal (BIST) technology. This paper presents a review of the available literature covering various types of BIST technologies and their applications in terms of structural design and architectural integration. The review covers detailed description of BIST systems using air, hydraulic (water/heat pipe/refrigerant) and phase changing materials (PCM) as the working medium. The fundamental structure of BIST and the various specific structures of available BIST in the literature are described. Design criteria and practical operation conditions of BIST systems are illustrated. The state of pilot projects is also fully depicted. Current barriers and future development opportunities are therefore concluded. Based on the thorough review, it is clear that BIST is very promising devices with considerable energy saving prospective and building integration feasibility. This review shall facilitate the development of solar driven service for buildings and help the corresponding saving in fossil fuel consumption and the reduction in carbon emission.

Thermodynamic analysis of CO2 transcritical two-stage compression refrigeration cycle systems with expanders

ABSTRACT The key to improving the efficiency of carbon dioxide (CO2) transcritical refrigeration systems is to select an appropriate two-stage compression system. In this study, the thermodynamic performance of six different CO2 transcritical two-stage refrigeration systems with expanders is compared. The design parameters are analysed to determine their respective impacts on the system coefficient of performance (COP) in order to ascertain which system configuration produces the maximum system COP. It is concluded that the system with a two-stage expander and incomplete intercooling has great advantages in terms of performance and equipment selection for the high-pressure stage in certain circumstances. Under the given conditions, the design parameters including the evaporating temperature, the condensing temperature, the compressor efficiency of the high-pressure stage, and the compressor efficiency of the low-pressure stage impact on the system performance. It should be noted that the gas cooler outlet temperature and the compressor efficiency of the low-pressure stage are the major factors which affect the efficiency of the system; thus, the system design and equipment selection should minimise the gas cooler outlet temperature and select a high-efficiency compressor for the low-pressure stage to ensure efficient and safe operation.