William W. S. Charters

Solar heat pump systems for domestic hot water

Vapour compression heat pumps can upgrade ambient heat sources to match the desired heating load temperature. They can offer considerable increase in operational energy efficiency compared to current water heating systems. Solar heat pumps collect energy not only from solar radiation but also from the ambient air. They can operate even at night or in totally overcast conditions. Since the evaporator/collector operates at temperatures lower than ambient air temperature it does not need glazing or a selective coating to prevent losses. Currently, however, they are not used much at all in domestic or commercial water heating systems. In this paper comparison is made of a conventional solar hot water system, a conventional air source heat pump hot water system and a solar heat pump water heating system based on various capital city locations in Australia. A summary is given of specific electricity consumption, initial and operating costs, and greenhouse gas generation of the three systems dealt with in this paper. The ultimate choice of unit for a particular location will depend heavily on the solar radiation, climate and the local price paid for electricity to drive or boost the unit chosen.

An ice thermal storage computer model

Abstract In hot humid countries such as Thailand, air conditioning plant is installed in most commercial and industrial buildings. A conventional air conditioning system, which is normally operated when cooling is required, is the most favored option. Ice thermal storage on a large scale, used to provide a cool reservoir for use in peak periods, is however an attractive financial option for large buildings to supply coolness. There are two means of operating ice thermal storage systems, namely full storage and partial storage. In this paper, a computer model has been developed in order to compare energy use in conventional air cooling systems and ice thermal storage systems. Under Thailand electricity tariff rates, the results from the simulations show that the full ice thermal storage can save up to 55% of the electricity cost required for cooling per month when compared with the conventional system. It is also found that using full storage option can reduce the total energy consumption by 5% for the selected building.

Thermal simulation of a passive solar house using a trombe-michel wall structure

Abstract Since the construction in 1967 in France of the first house with a “Trombe Wall” there has been continuing interest on a world-wide basis of the potential of passive solar systems. The integration of the absorber as part of the building structure can lead to substantial reduction in building costs of active solar systems composed of absorbers, pumps, controls, etc.; and the simplicity of the system ensures that the structure will require no more maintenance than a conventional house. The prototype of this system was built in 1967 and later designs were constructed in 1974 in the French Pyrenees. There is continuing programme of research and development into passive solar systems sponsored by the French Government through CNRS. At the University of Melbourne we have investigated over the past 2 yr the feasibility of applying such a passive solar system to Australian conditions. We have studied by computer simulation the thermal performance of the wall structure during the worst month of a Melbourne winter when equipped with a solar wall collector. This is essentially an adaptation of the French concept to conventional Australian building methods. It is interesting to note, that this type of system readily adapts itself to a modular construction approach with factory made solar walls pre-assembled and delivered complete for site installation. The solar contribution to the heating load in winter conditions appears to be of the order of 40 per cent, as predicted from the theoretical computer models and confirmed from the performance data published from the monitored French houses at Odeillo.

Natural working fluids for solar-boosted heat pumps

The option of using natural working fluids as a substitute of R-22 for solar-boosted heat pumps depends not only upon thermal performance and hazardous rating but also on potential impacts on the environment. This paper presents the comparative assessment of natural working fluids with R-22 in terms of their characteristics and thermophysical properties, and thermal performance. Some justification is given for using natural working fluids in a solar boosted heat pump water heater. The results show that R-744 is not suitable for solar-boosted heat pumps because of its low critical temperature and high operational pressures. On the other hand, R-717 seems to be a more appropriate substitute in terms of operational parameters and overall performance. However, major changes in the heat pumps are required. R-290 and R-1270 are identified as candidates for direct drop-in substitutes for R-22.

Natural convection phenomena in inclined cells with finite side-walls—A numerical solution☆

Abstract The effects of Rayleigh number, aspect ratio and angle of inclination on the heat transfer through an inclined air-cell are studied via the numerical solution of the relevant two dimensional governing equations. This is done for two side wall boundary conditions, namely that of perfectly insulating and infinitely conducting side walls. In addition, the effects of a finite thickness wall with a finite value of thermal conductivity is studied, by solving the two dimensional conduction equation in the wall and matching the values of temperature and heat flux at the common boundaries between the two regions. It is shown that the perfectly in sulating and infinitely conducting boundary conditions are the two extremes of the real case and, depending on the values of the fluid parameters and the value of the wall thermal conductivity, the two fixed boundary conditions can be either accurate or inaccurate representations of the real case. A discussion of the relevance of this work to solar absorbers is included, with the major conclusion being that, depending on the aspect ratio of the cell used, cellular structures can be effective in reducing convective losses in inclined absorbers.

Electrical and engine driven heat pumps for effective utilisation of renewable energy resources

Much of the energy used for domestic, commercial and industrial purposes is to provide efficient and effective heating of conditioned spaces and for specialist niche applications in process heat systems. Vapour compression heat pumps driven by electric motors or engines provide the real capability of upgrading low temperature sources of ambient and waste heat to match the desired load temperatures in such heating applications. Major source of ambient heat stem from the storage of solar energy in the ground, in lakes and rivers, and in atmospheric air. Heat pumps can therefore be used to effectively harness indirectly the daily solar radiation input. In addition many industries have major sources of waste low grade heat in the form of air or water discharged from the industrial process heat stream. Heat pumps are generally formally classified therefore as air source, ground source or water source units although there has also been considerable interest recently in hybrid units combining the attributes of two or more of these specific types mentioned above.

Solar energy: Current status and future prospects

Abstract Although solar radiation has been used extensively ever since mankind moved from the hunter-gatherer mode to one of settlement agriculture it is only in recent decades that major scientific and technical effort has been dedicated to harnessing the power of the sun. Once solar radiation has been absorbed by a suitable surface and converted to heat it is possible to use this for a wide range of heating purposes at varying temperature levels. This use is often given the generic term of solar thermal systems. Solar radiation can also be used directly via the photovoltaic effect to produce electricity or indirectly through the solar thermal electric conversion process.

Saturation property equations for R22

Abstract The fundamental relations for the thermophysical properties of refrigerant R22 over an extended range of saturation temperatures have been documented in this Short communication. These empirical expressions, which are associated with saturated liquid and saturated vapour states, speed up the computations involved in system modelling and computer simulation of vapour compression units.

Solar vaccine storage units for remote areas

Abstract Recent cost reductions in photovoltaic arrays have once again stimulated interest in the possibilities offered by photovoltaic powered vapour compression units. These units are now becoming commercially available with either d.c. or a.c. power drive to the refrigeration compressor. Extensive field testing of solar photovoltaic refrigerators has been carried out, under the auspices of the World Health Organisation (WHO, Geneva), with the ultimate aim of their wide-scale use in developing countries for essential purposes such as vaccine serum storage at medical clinics in remote regions. Indications are that there is a large market demand world-wide for such small-scale medical units. A technical and economic appraisal of the current state-of-the-art technology will be presented in this technical note following on from some recent work carried out at the University of Melbourne.

Property equations for saturated water

Abstract Due to increased use of computers, it is becoming more convenient, for many engineering applications, to formulate the thermal and transport characteristics of the common working fluid, water, at the saturation state.