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

https://doi.org/10.1016/S1359-4311(03)00056-5Get rights and content

Abstract

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.

Introduction

Heat pumps are devices designed to utilise low temperature sources of energy which exist in atmospheric air, lake or river water and in the earth. All of this energy is made available initially by solar radiation reaching the earth’s surface and its use constitutes therefore an indirect use of solar energy. Because of this design characteristic heat pumps can also be operated using “waste heat” from commercial and industrial processes and thereby upgrading this to the required temperature level for some thermal process operation [1].

In the process of upgrading the low temperature heat it is necessary to provide an electrical or mechanical power input to the heat pump compressor in order to raise the pressure and hence the temperature of the working fluid––see Fig. 1, Fig. 2.

All vapour compression heat pumps use refrigerant working fluids capable of evaporating and condensing at the appropriate temperature levels of the heat source and the load. With the recent attention given to global warming and the destruction of the ozone layer in the upper atmosphere it is imperative to ensure that all future refrigerant fluids satisfy the criteria of low global warming potential (GWP) and low ozone depletion potential (ODP). This has caused major research into new fluids moving from chlorofluorocarbons (CFCs) through hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) to pure hydrocarbons (HCs). The focus is on efficient and safe operation of these new fluids and the older natural fluids such as ammonia for heat pump units under both domestic and commercial conditions of operation.

For effective design purposes all heat pump systems should be rated to include heating and cooling capacity limits, and realistic measurements of system coefficient of performance (SCOP) on a seasonal or annual basis. SCOP is defined as the ratio of the useful thermal energy output to the total energy input of the system. Often in practice manufacturers give only the compressor coefficient of performance (CCOP) and that often on an instantaneous and not time average basis. CCOP is defined as the ratio of the useful thermal energy output to the compressor energy input.

Hybrid heat pump systems currently under investigation include ground source/air source units, and solar assisted and solar boosted air source and water source units.

The potential for energy conservation in the built environment and the efficient utilisation of energy in cascaded heat recovery systems in industrial process heat application is immense. In fact the International Energy Agency has several times identified the heat pump as the single most energy conservation technology on a world scale. There are also many opportunities for melding together the efficiency of heat pump systems with the inherent environmental advantages of renewable technologies.

Section snippets

Working fluids

Originally heat pumps were designed to run on a range of natural working fluids such as ammonia and carbon dioxide but the introduction of the specially designed refrigerant fluids known as CFCs soon dominated the refrigeration, air conditioning and heat pump industry. These fluids had tailor-made properties of a thermophysical nature which made them unique in their usefulness. It is only in recent times that the knowledge of their ozone destruction capabilities has come to the fore with their

Operational characteristics and performance parameters of heat pumps

In order to quantify the operational characteristics of vapour compression heat pumps it is important to define the terminology carefully used to delineate the various paths taken in an overall energy flow diagram. As all such units use a high level power input this has to be carefully measured and minimised in practice to increase the overall unit effectiveness often quoted as the coefficient of performance (COP) based either on the compressor alone or on the total system.

Many manufacturers

Temperature lift (Δt)

The difference between the condensing temperature and the evaporating temperature of the working fluid on the heat pump cycle is known as the “temperature lift”. The less the temperature lift is the less the work of compression per unit mass flow will be. For any practical applications the temperature lift is greater than the temperature difference from the load temperature to the source temperature due to the large finite temperature drops needed in practice in the evaporator and condenser to

Solar boosted and solar assisted heat pumps

From the previous discussion it is evident that increasing the evaporator temperature under given conditions will reduce the cycle temperature lift and thereby increase the COP. One of the ways this can be done is by having solar energy either directly on the evaporator (solar boost) or indirectly through a conventional solar collection system (solar assisted) [4]. Both of these systems have been used in industrial process heat and building heating and cooling applications over the last 25

Market opportunities for heat pumps

Recently published work [5] gives an indication of current future sizes for a range of domestic, conventional, and industrial heat pumps and gives a good indication of their economic viability.

Any such estimation of future market potential relies heavily on a prediction of conventional fuel and electricity costs and pricing policies. Full technical and economic analyses are required in order to estimate the rate of return on investment on these new technologies.

Conclusions

Major advances have been made in vapour compression heat pump technologies over the last 20–30 years. Compressor failures are now extremely rare and the system reliability and control is in general very good.

Heat pumps in their various and diverse forms allow one to harness solar energy in a cost effective fashion which has already been stored in the atmosphere and the biosphere and can be viably tapped [6].

References (6)

There are more references available in the full text version of this article.

Cited by (0)

View full text