Ambient Energy Fraction of a Heat Pump

Transcription

1 Ambient Energy Fraction of a Heat Pump u Aye, R. J. Fuller and.. S. Charters International Technologies Centre (IDTC) Department of Civil and Environmental Engineering The University of Melbourne Vic 3010 AUSTRAIA lua@unimelb.edu.au Abstract Heat pumps are renewable energy systems which offer the possibility of reducing energy consumption significantly. This paper presents fundamental principles and applications of heat pumps in general. A new term ambient energy fraction for heat pump energy analyses is introduced for comparing an electric heat pump system with other systems for the same thermal output. The concept and the use of this term are discussed with numerical examples. The ambient energy fraction of an electric heat pump depends on the coefficient of performance (COP) of the heat pump and the power plant efficiency based on the primary energy used. It is highly dependent on the type of power plant used to generate the electricity to drive the heat pump compressor. 1. INTRODUCTION Although most people are familiar with the common household refrigerator the concept of using such a device to provide heating rather than cooling is less widely understood. However the conception of the heat pump itself dates back to the middle of the 19 th century (Thompson 1852 cited in Gac et al. 1988). Presently about 130 million heat pumps with an annual thermal output of 1300 Th are in operation worldwide, reducing carbon dioxide emissions by about 0.13 Gt annually (Halozan & Rieberer 2004). A heat pump is a thermodynamic system whose function is to heat, at the required temperature with the aid of heat extracted from a source at lower temperature (Gac et al. 1988). 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. These sources are referred to as ambient energies. Heat pump 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 operations (Mc Mullen & Morgan 1981). Major advances have been made in heat pump technologies over the last years. Heat pumps in their various and diverse forms allow one to harness solar energy in a cost effective manner which has already been stored in the atmosphere and biosphere. This paper introduces ambient energy fraction to make a fare systematic comparison with other systems which provide the same thermal output. The concept and the method of estimating the ambient energy fraction are discussed. The relationship between the ambient energy fraction, the heating coefficient of performance (COP h ) and primary energy ratio (PER) is also presented Fundamentals There are many types of heat pumps available but in this paper we will restrict ourselves to consideration of the vapour compression units in which a suitable working fluid capable of evaporation and condensation as shown in Figure 1. To carry out these processes any practical heat pump uses four separate components: compressor, condenser, expansion device, and evaporator as illustrated in Figure 2.

2 Pressure (p) 3 condensation 2 throttling 4 evaporation 1 compression Specific Enthalpy (h) Figure 1. Pressure enthalpy diagram for ideal heat pump cycle High temperature, Heat delivery iquid line High pressure liquid Condenser High pressure side Discharge line High pressure vapour Thermostatic expansion valve Compressor ow pressure mixture of liquid & vapour ow pressure side Evaporator ow temperature, Ambient heat source AIR / GROUND / ATER Suction line ow pressure vapour Figure 2. Schematic of heat pump circuit (Source u Aye & Charters 2003) The heat pump is essentially a device used to take in energy from a low grade source, R, i.e. one at low temperature and to upgrade this thermal energy to a more suitable temperature level by providing a mechanical energy input, (Figure 3). High temperature sink = R + Heat pump unit R ow temperature source Figure 3. Energy flow from low temperature source to high temperature sink Renewable Energy for a Sustainable Future A challenge for a Post Carbon orld ANZSES

3 The heat pump must, by the principle of conservation of energy, provide more energy ( = R + ) at the high temperature side than is used to drive the mechanical compressor. The ratio of delivered output heat energy to input mechanical work is known as the heat pump heating coefficient of performance (COP h ) and must always theoretically be greater than unity. R + R COP h = = = 1+ > 1 (1) The heat pump in most cases upgrades free heat from the environment (air, water, ground) and from waste heat, is a major source of renewable energy. It should be noted that renewable energy gained by the heat pump is R = Solar Energy and Heat Pumps Although it is fair to say that all heat pumps are solar units as the low temperature energy source whether it be ambient air, river or lake source water, or earth ground effect is always indirectly solar heated this is rather like saying that all houses are effectively solar heated. Just as in practice most architects and engineers identify solar houses as one in which particular attention has been paid to maximising the useful solar input so we distinguish here solar heat pump units as ones in which a deliberate attempt has been made to achieve improved performance levels through utilising a solar energy input. If the solar input is used indirectly to raise the evaporator temperature the unit is termed solar assisted heat pump (SAHP) (Figure 4) and if the evaporator becomes an evaporator/collector unit for direct capture of the solar radiation we term it solar boosted heat pump (SBHP) (Figure 5). One of the early attempts at harnessing solar energy to improve the overall performance of a heat pump system has been described by Sporn and Ambrose (1955). Charters and Taylor (1976) reported on an SBHP using a flat-plate collector without a cover as the evaporator. This type of system is also named as direct expansion solar collector-heat pump system. Unlike thermosyphon solar water heaters, solar heat pump systems provide a real capability of upgrading low-grade energy sources from the surroundings as well as using solar energy (Charters et al. 1980; u Aye & Charters 2003). Compressor Solar array Thermal storage Evaporator Condenser Supply Return + TXV iquid receiver Figure 4. Solar assisted heat pump (SAHP) schematic circuit (Source 2005) Evaporator absorber TXV Condenser iquid receiver Supply Return Compressor Figure 5. Solar boosted heat pump (SBHP) schematic circuit (Source 2005) Renewable Energy for a Sustainable Future A challenge for a Post Carbon orld ANZSES

4 1.3. Applications Heat pumps have been used for domestic, commercial and industrial applications. Domestic applications are: Provision of space heating; Provision of hot water; and Swimming pool heating Commercial and Industrial applications are: Space heating; ater heating; Swimming pool heating; Drying and dehumidification; Evaporation and boiling; and Desalination Primary Energy Ratio (PER) The coefficient of performance (COP) provides a measure of the usefulness of the heat pump system, in producing large amounts of heat from a small amount of work. It does not, however express the fact that energy available as work is normally more useful than energy available as heat. In order to assess different heat pump systems using compressor drives from different fuel or energy sources the primary energy ratio (PER) is applied. The PER takes into accounts not only the heat pump COP but also the efficiency of conversion of the primary fuel into the work which drives the compressor. PER is defined as follows (Reay & Macmichael 1979): Useful heat delivered by heat pump PER = (2) Primary energy consumed The drive energy of heat pumps is most commonly electricity. Ideally a heat pump where free work is available should be contemplated e.g. wind or water power (Reay & Macmichael 1979). Consider an electric heat pump powered by a conventional power plant fuelled by a non-renewable energy (Figure 6). E pe Heat pump Power plant R Q Figure 6. Energy flow of a conventional electric heat pump PER COP = = = h ηpp (3) Epe Epe Renewable Energy for a Sustainable Future A challenge for a Post Carbon orld ANZSES

5 here = load and E pe = primary energy used by the heat pump system. The power plant efficiency, η pp, is up to 58% for oil or gas-fired combined-cycle power plants currently available on the market. The PER is equal to the COP for direct power generation from renewable ambient energy sources such as solar and wind (Halozan & Rieberer 2004). This concept is illustrated in Figure 7. E re Heat pump Renewable power R Figure 7. Energy flow of a renewable electric heat pump The amount of renewable or ambient energy spent (used up) to produce work for the heat pump is equal to the amount work (i.e. E re = ). For this case by definition η pp = 1 (Halozan & Rieberer 2004). It should be noted that unlike the losses in a fossil fuel power plant the unused ambient energy passing through the renewable power plant is still in the form of ambient energy and it is available to be used. 2. AMBIENT ENERGY FRACTION The term solar fraction is widely understood, accepted and used in the solar energy field. It is defined as the fractional reduction of purchased energy when a solar energy system is used (Duffie & Beckman 1991). It is the fraction of the load contributed by the solar energy, which can be calculated by Equation (4). f E = = s (4) here = the load, E = the auxiliary energy supplied to the solar energy system, and s = the solar energy delivered. Similar to the solar fraction of a solar system the term ambient energy fraction of a heat pump can be defined as the fraction of the load contributed by the ambient energy, which may be calculated by Equation (5). fae = E pe E pe 1 1 = 1 = 1 = 1 (5) PER COPh ηpp Figure 8 illustrates the relationship between the ambient energy fraction and the primary energy ratio. Renewable Energy for a Sustainable Future A challenge for a Post Carbon orld ANZSES

6 Ambient energy fraction ( fae ) Primary energy ratio (PER ) Figure 8. The ambient energy fraction verses the primary energy ratio Table 1 shows the ambient energy fractions of a heat pump, which has a heating COP of 3.5, for various electric power plants. It can be clearly seen from Table 1 that for the same heat pump the ambient energy fraction may vary from 1 % to 71 % depending on how electricity is generated for driving the heat pump compressor. The ambient energy fraction for the renewable electricity driven heat pump is the highest (71 %). The renewable energy used is only 29 % (i.e 100 % 71 %) of the total thermal load. The energy used of the brown coal electricity driven heat pump is 99 % of the total thermal load (i.e MJ of brown coal energy is required for 1 MJ of thermal load). Table 2 shows the effect of heat pump COP on the ambient energy fraction of a heat pump powered by renewable. Table 1. Ambient energy fractions of a heat pump powered by typical power plants Power plant Brown Nuclear Black Gas-fired Renewable coal coal combined-cycle Power plant efficiency, η pp * 0.29 ** 0.33 * 0.35 ** COP of heat pump Primary energy ratio, PER Ambient energy fraction, f ae * Typical Australian data (Source: Taylor & abson 1997) ** Typical European data (Source: Halozan & Rieberer 2004) Table 2. Effect of COP on Ambient energy fractions of a heat pump powered by renewable COP of heat pump Renewable power plant efficiency, η pp Primary energy ratio, PER Ambient energy fraction, f ae CONCUSIONS The definition and concept of a new term ambient energy fraction have been presented. The following conclusions may be drawn: The ambient energy fraction can be used to systematically compare various heat pump systems. Renewable Energy for a Sustainable Future A challenge for a Post Carbon orld ANZSES

7 The ambient energy fraction of an electric heat pump depends on the COP and the power plant efficiency based on the primary energy used. The ambient energy fraction highly dependent on the type of power plant used to generated the electricity to drive the heat pump compressor. Heat pumps driven by renewable electricity offer the possibility of reducing energy consumption significantly. 4. ACKNOEDGMENTS The authors would like to thank Ken Guthrie, Sustainable Energy Authority Victoria for the discussion with us about the energy accounting issues related to heat pumps during the last ANZSES conference. The secretarial support from the International Technologies Centre (IDTC), Department of Civil and Environmental Engineering, The University of Melbourne is also acknowledged. 5. REFERENCES Charters,.. S. and Taylor,. E. (1976), Some performance characteristics of a solar boosted heat pump. In Proceedings of IIR Conference: Towards an Ideal Refrigerated Food Chain, pp , Melbourne, Australia. Charters,.. S.; de Forest,.; Dixon, C.. S. and Taylor,. E. (1980), Design and performance of solar boosted heat pumps. In Australian and New Zealand Solar Energy Society Annual Conference, Melbourne. Duffie, J. A. and Beckman. A. (1991), Solar Engineering of Thermal Processes, 2nd ed., A iley- Interscience publication, NY. Gac, A., Vrinat, G., Blaise, J.-C., Camous, J.-P., and Fleury, M. (1988), Guide for the Design and Operation of Average and arge Capacity Electric Heat Pumps, International Institute of Refrigeration, Paris. Halozan, H. and Rieberer, R. (2004), Energy-efficient heating and cooling systems for buildings IIR Bulletin, vol. XXXI, no , pp u Aye and Charters,.. S. (2003) Electrical and engine driven heat pumps for effective utilisation of renewable energy resources, Applied Thermal Engineering, vol. 23, no. 10, pp u Aye, Charters,.. S. and Chaichana, C. (accepted 13 April 2005), Solar boosted heat pump, Progress in Solar Energy Research, Nova Science Publishers, Inc., NY, pp Mc Mullen, J. T. and Morgan, R. (1981), Heat Pumps, Adam Hilger td, Bristol, UK. Reay, D. A. and Macmichael, D. B. A. (1979), Heat Pumps Design and Applications A Practical Handbook for Plant Managers, Engineers, Architects and Designers, Pergamon press, Oxford, UK. Spon, P. and Ambrose, E. R. (1955), The heat pump and solar energy, Proceedings of orld Symposium on Applied Solar Energy, Phoenix, Arizona. Taylor, M. and abson, S. (1997), Profiting from Cogeneration, DPIE: Commonwealth Department of Primary Industries and Energy/Australian Cogeneration Association, Canberra, Australia. Thomson,. (ord Kelvin) (1852), On the economy of the heating and cooling of buildings by means of currents of air, Proceedings of the Glasgow Philosophical Society, vol. V, 3 December Renewable Energy for a Sustainable Future A challenge for a Post Carbon orld ANZSES