Heat Pumps

Reducing Losses – Increasing Efficiency

Hermann Halozan

Institute of Thermal Engineering, Graz University of Technology,

Inffeldgasse 25 / B, A-8010 Graz, Austria 

1. INTRODUCTION

European governments have committed to significantly reduce the carbon footprint of new and existing homes. It is therefore generally accepted that: increasing the uptake of renewable energy systems and improving the thermal performance of both new and existing homes is critical if Europe is to achieve ambitious emission targets.

Heat pumps offer the possibility of reducing energy consumption significantly, mainly in the building sector. Taking different applications of heat pumping technologies several items have to be taken into consideration like drive energy, design of the unit, integration into a system and the control strategy.

The efficiency of the unit commonly expressed by the COP, the coefficient of performance. This COP depends on the refrigerant selected and on the components used like the compressor, the size and the design of condenser and evaporator, the flow sheet – single stage, two stage, economiser or cascade – and the internal cycle control. The choice of the refrigerant is most commonly a compromise between efficiency and cost, smaller equipment using a high-pressure working fluid can reduce the cost, a working fluid with low discharge temperatures can avoid a two-stage system. With highly efficient systems the advantages of thermodynamic heating and cooling can be demonstrated and  used for reducing the energy demand significantly. Heat pumps are one of the key technologies for energy conservation, increasing the share of renewable energy used and reducing CO2 emissions.

2. HEAT PUMPS

The general term heat pumping technologies is used for processes in which the natural heat flow from a higher to a lower temperature level is reversed by adding high value energy, i.e. exergy. In Europe the term heat pump has used for heating-only units with the heat sources outside air or exhaust air from the ventilation system, ground and ground water, combined with hydronic heat distribution systems (Gilli, Halozan 2001). However, caused by the better thermal insulation of the building shell cooling becomes more and more common, like in Japan and in the US.

The seasonal performance factors of heat pump systems are usually poor compared to the steady state COP. Taking a monovalent air/water heat pump integrated directly into the return line of a low-temperature distribution system, the SPF which can be achieved will be in the range of 2.2 to 2.4. If one models this system, using a heat pump with the same heating capacity, but assuming ideal capacity control and operation with Carnot efficiency, the SPF will become 8.35. This means that the efficiency factor of the actual system is 0.27. This is a rather poor result. The reasons for this poor efficiency factor are: 

• losses of the heat pump unit itself, realizing the real process and not the ideal Carnot process, with non-isentropic compression, electrical losses, temperature drops at the condenser and the evaporator, pressure drops, etc.,
• losses because of transient conditions in the heating system during operation,
• losses caused by frosting and defrosting, and
• mixing losses caused by heat pump outlet temperatures exceeding the necessary supply temperature.

To achieve high efficiency heat pump systems, all these losses have to be minimized or even avoided.

A good way of analysing the steady-state losses of a heat pump unit is to make an efficiency factor plot. Fig. 1 shows such a plot for an air/water heat pump unit; the efficiency factor depends on the heat source inlet temperature and the heat pump outlet temperature (Halozan, Katona, Gilli1988).

The losses which affect the efficiency factor are caused by:

• the compression, that is the compressor used;
• the utilized refrigerant, which forces variations from the ideal Carnot cycle;
• the electric drive;
• temperature differences between condenser und heat pump outlet;
• temperature differences between heat source and evaporator;
• temperature losses due to suction gas superheating;
• internal pressure drops; and
• parasitics losses from fans etc.

Each heat pump has a characteristic optimum of the efficiency factor, and the conditions where the maximum efficiency factor occurs are the operating conditions where the unit is best suited. Fig 3.2 shows an air/water heat pump better suited for operation in a low-temperature system. The maximum of the efficiency factor – in this case 0.428 – is located at 0 C outside air temperature and a heat pump outlet temperature of 42 C.

Fig. 1: Efficiency plot of a 8 HP Air/Water Heat Pump

During  the operation of an heat pump in s system some dynamic losses are introduced,

• Cycling Losses due to on/off operation, and
• Mixing losses caused by heat pump outlet temperatures higher than required.

Mixing losses occur when a heat’ pump is charged with a constant flow rate and the heat pump capacity does not match with the load. A good example is the direct integration of a heat pump into a hydronic system. In this case the heat pump is sized for a certain balance temperature. At temperatures higher than this balance temperature, the heat pump outlet temperature is higher than the necessary supply temperature. This means higher condensing temperatures than necessary and a lower COP. Temperatures and capacities of such a System are shown in Fig. 2.

Fig. 2: Air/Water Heat Pump directly integrated into a Hydronic System

To reduce these mixing losses various measures are possible such as the installation of a storage tank. By means of this storage tank it is possible to decouple the flow rates through the heat pump and the distribution system. The heat pump can be operated with a higher flow rate and heat pump outlet temperatures can be reduced (see Fig. 3.). The disadvantages of this system are that it is expensive and that the control is complicated and also expensive.

Fig. 3: Air/Water Heat Pump integrated into a Hydronic System by means of a Store – Flow Rate 2.2

A further possibility is to use a heat pump unit with capacity control by means of capacity control in steps using a two-speed compressor (see Fig. 4). Mixing losses caused by high condensing temperatures and cycling losses can be reduced significantly.

Instead of one unit with capacity control two or even more units can be installed. In the case of three units arranged in parallel, the heat pump outlet temperatures are the same as for one unit. However, cycling is reduced and the SPF can be increased. The better way is to arrange the units in series (see Fig. 5). In. this case not only mixing losses can be reduced. If more than one unit works, the first unit acts as a preheater with a lower condensing temperature than the following units, and only the last unit has to work with the necessary supply temperature level.

Fig. 4: Air/Water Heat Pump with a two-speed compressor directly integrated into a Hydronic System

Fig. 5: Three Air/Water Heat Pump arranged in series directly integrated into a Hydronic System

The presently most popular method is to use an inverter-driven unit for reducing cycling losses as well as mixing losses (see Fig. 7). In this case capacity and load match over a wide range. Comparing the SPFs achieved with the above mentioned systems, the lowest SPF occurs in the first system, the highest in the system with three heat pump units arranged in series. The system with the inverter controlled heat pump is remarkably better than the first system, and considering the cost situation this system seems to be the most promising one for the future.

Integrating heat pump units into low-temperature distribution systems can mean bivalent operation (in case of retrofit) as well as monovalent operation. The maximum necessary supply temperature is always lower than the maximum heat pump outlet temperature. The only limit that can occur is the minimum heat source temperature suitable for the special heat pump unit. Fig 6 shows the situation for a 6 HP air/water heat without inverter. The SPF of this system is about 2.5, the energy share covered by the heat pump about 96 %, in practice this share means monovalent operation. The balance temperature of this system is about -12 C. At temperatures above the balance temperature, heating capacities do not match the load, and the heat pump outlet temperatures exceed the necessary supply temperatures remarkably. Fig. 7 shows the same system with a 6 HP unit with inverter with a frequency range of 1:5. The SPF rises to 3.1, the share of energy covered by the heat pump is again 96 %. Mismatch of heating capacity and load is reduced drastically. Sized for monovalent operation they use the highest frequency – with the lowest efficiency – for peak load operation only. The maximum of the operating hours is covered with moderate frequencies.

Fig 6: 6 HP single-speed air/water heat pump, 12 kW building, floor heating system

Fig. 7: 6 HP inverter-driven air/water heat pump, frequency range 1:5,12 kW building, floor heating system

With the water/water heat pump units even better improvements can be achieved. Fig 8 shows the situation for a 3 HP water/water heat pump without inverter integrated into the 12 kW house with low-temperature radiator system. The SPF is about 3.2, the energy share covered by the heat pump is 100 %. The balance temperature of this system is about -20 C. At temperatures above this balance temperature, heating capacities do not match the load, and the heat pump outlet temperatures exceed the necessary supply temperatures remarkably. Cycling and mixing losses lower the SPF. With an inverter-driven unit the results ca be improved significantly (see Fig 9).

3. BUILDINGS

In general, new buildings get a better thermal insulation and the heat loads are reduced significantly. This means that even in “cold” climates (design temperatures -12°C, heating degree days 3500, heating period length 200 days) buildings with specific heat loads of  60 W/m2 and can be heated by ground-source heat pump systems achieving SPFs of 4 up to 6 (see Fig. 10).

Fig. 8: 3 HP single-speed water/water heat pump, 12 kW building, floor heating system

Fig. 9: 3 HP inverter-driven water/water heat pump, frequency range 1:5, 12 kW building, floor heating system

Fig. 10: Ground-source heat pump systems – horizontally installed collector, bore hole heat exchangers

A further step has been already realised in the so called passive houses: The transmission losses through the building envelope are in the range of 15 W/m2 (Fig 11).

Fig 11: Passive house heating, cooling, hot tap water system

A step further are “Net Zero Energy Houses” or even “Energy Plus” Houses. They are additionally equipped with a photovoltaic panel producing during the year the electricity needed by the building (Fig. 12).

Fig. 12: Net-zero energy building, Energy plus building

4. CONCLUSIONS

Heat pumps are an old technology, which has not been extensively used as long as both energy prices and the efficiency of electricity generation have been low. The oil crises have changed this situation, and now Kyoto is a further reason for the increasing market deployment of this technology. Based on recent developments, the following conclusions can be drawn:

• Heat pumps offer the possibility of reducing energy consumption significantly, mainly in the building sector, but also in industry. Basic second law thermodynamics show the advantages: while a condensing boiler can reach a primary energy ratio (PER) of 105 % (the theoretical maximum would be 110 % based on the lower calorific value), heat pumps achieve 200 % and more, with hydro or wind energy even 400 % and more.
• The drive energy is most commonly electricity, and for the future improved power generation systems based on renewable and fossil fuels have to be taken into consideration. The efficiency of gas-fired combined-cycle power plants available on the market is presently about 60 %, with oil as fuel similar values are possible.
•  Ground-source (“geothermal”) heat pumps combined with low-temperature heat distribution systems achieve seasonal performance factors (SPFs) of 4 and higher, which means PERs of 220 to 280 %.
• Outside air heat pumps start dominating the market caused by  lower investment cost, especially in the retrofit market.
• Direct-exchange ground-source heat pumps already achieve SPFs between 4 and 7 !, if building standards are kept and the overall system design has been made carefully.
• The choice of refrigerants presently in use – R-407C, R-410A and propane, for large units additionally R-134a and ammonia, is motivated by efficiency, reliability, environmental considerations, safety and regulations.
• The next fight on refrigerants has already started: HFOs versus R32 and the so called naturals like ammonia, the hydrocarbons and CO2 (Halozan, Rieberer 2006).
• With highly efficient systems the advantages of thermodynamic heating and cooling can be demonstrated and used for reducing the energy demand, increasing efficiency and increasing the share of renewables (Halozan 2010).

The potential for reducing CO2 emissions assuming a 30 % share of heat pumps in the building sector using technology presently available is about 6 % of the total world-wide CO2 emission. With advanced future technologies in power generation, in heat pumps and in integrated control strategies up to 16 % seem to be possible. Therefore, heat pumps are one of the key technologies for energy conservation and reducing CO2 emissions.

REFERENCES

1. Gilli, P.V., Halozan, H. (2001), Heat Pumps for Different World Regions – Now and in the Future, Proc. 18th WEC Congress, Buenos Aires, Argentina.
2. Halozan, H. (2010) Limits of Heat Pumps in LowEx Design, Proceedings ECBCS Annex 49 Conference The Future for Sustainable Built Environments with High Performance Energy Systems, 19th – 21st October 2010, Munich, Germany
3. IEA HPC (2010), Annex 29 Ground Source Heat Pumps – Overcoming Market and Technical Barriers, IEA HPC, Sittard, Netherlands, 2010
4. Halozan, H., Katona, O., Gilli, P.V. (1988) Inverter-Driven Heat Pumps, IEA HPC, Sittard, Netherlands, September 1988