Module 13: Design of air source heat pump systems for heating and hot water

The UK has a challenging and legally binding target of generating 15 per cent of its energy from renewables by 2020. The government’s Renewable Energy Strategy has recognised the potential of commercial buildings to contribute to this shift, and the potential for air source heat pumps is starting to be recognised. In December 2008, the European parliament adopted an EU directive on the Promotion of Renewable Energy Sources, which expanded its definition of renewable energy sources to include air and water source heat pumps, in addition to ground source heat pumps. Now all three technologies are being promoted as part of EU policy to get member states to increase their use of renewable energies and this ruling will make its way into UK legislation in 2010.

Posted in February 2010

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This CPD article examines some practical issues in selecting and applying air to water heat pumps. For an introduction it would be useful to refer back to the May 2009 CPD article, where the basic operating principles of a heat pump are described, with its related performance characteristics. Note that efficiency of a heat pump is defined using the Coefficient of Performance(COPh) (heating) or Energy Efficiency Ratio(EER) (cooling).

COPh = Useful Heating Capacity (kW) / Power Input to drive the system (kW)

Values between 2 and 5 are attainable for air source heat pumps producing hot water. The efficiency is principally a function of the heat pump’s operating temperatures, the evaporating and condensing temperatures and these will be a function of the air intake temperature and the hot water temperature required. As far as wanting to improve efficiency is concerned, if the evaporating temperature could be raised by 1K and the condensing temperature reduced by 1K, performance of the system will increase quite considerably, by between 4 and 8%. Figure 1 shows the variation in COPh for a typical air-to-water heat pump in the capacity range 3 to 15kW. Once the air and water temperatures are fixed, improvement in efficiency is achieved by heat exchanger design and selection, plus developments in compressors and controls.

Fig. 1: Performance of Air-Water heat pump for hot water temperatures of 35ºC and 50ºC

Heat pumps are most typically applied for space heating, space cooling and also for the production of stored sanitary hot water.

For heating operation, heat pumps will always optimise their efficiency at low water temperatures and are most effectively installed in conjunction with low temperature heat distribution systems, typically underfloor heating or fan convectors. If conventional radiators are used, these need to be suitably oversized in order to allow effective operation at lower water flow temperatures. Radiator manufacturers provide tables to calculate the radiator output across a range of mean water flow temperatures.

In the case of underfloor heating, designers should ensure that the underfloor pipe matrix is designed with low temperature operation in mind (ideally 35ºC – 40ºC); this is largely a function simply of pipe spacing.

A widely held misconception about heat pumps is that they are only suitable for space heating and even then, only at low water temperatures. However, heat pumps also have the ability to provide plentiful hot water, either as the sole water heating appliance or, if preferred, in conjunction with another heat source such as solar or a fossil fuelled boiler.

Fig. 2: Selection procedure, step 1

As hot water flow temperatures increase, heat pump efficiency (Coefficient of Performance) does drop, but even at a relatively low COPh of 2, this is still equivalent in terms of CO2 emissions to a gas powered system. A well-designed and sized heat pump will normally be operating above this value, even for hot water production. New generations of air source technology available from some manufacturers now offer performance that is comparable to ground source, with high COPs at low ambient temperatures. In addition, the incorporation of renewables brings other benefits. For example, a recent installation at a Travelodge used three 28kW air source heat pumps to pre-heat domestic hot water for the hotel, helping it to achieve the percentage of renewable energy required to meet the Merton Rule, as well as achieving compliance with Building Regulations Part L.

Defrosting

Specifiers must also consider the energy impact of an air source heat pump’s requirement to defrost its evaporator to remove ice build up at low ambient temperatures. This is most commonly achieved by the heat pump switching into reverse cycle operation. When this happens the outdoor heat exchanger (evaporator) becomes the condenser, with hot refrigerant used to remove the ice. When operating in this mode, electricity continues to be used by the compressor and heat is removed from the heat sink (that is the condenser temporarily becomes the evaporator). This is a fundamental reason why air source heat pumps should always be installed in conjunction with a buffer tank as this prevents heat being extracted from the buildings heating system when operating in the defrost cycle.

Typically the defrosting cycle takes place between ambient temperatures of approximately 10ºC and 0ºC. In the UK maritime climate where these reflect typical winter temperatures, it is therefore crucial that the efficiency of the defrost cycle is taken into consideration and that manufacturers quoted performance includes energy consumption of the defrost cycle.

Selecting a heat pump

For a new build the situation is straightforward. The design heating load is determined for a particular winter design ambient temperature. The type of heat emitter is decided (radiators, underfloor heating, fan coil, etc.), which determines the water temperature required. Underfloor heating and fan coils can operate at water temperatures of 35ºC, but radiators will need water at least at to 55ºC.

If a heat pump system is retrofitted into an existing building that had a gas/oil boiler system, the following factors have to be considered:-

  • The correct actual flow and return water temperatures have to be determined for each heat emitter (most likely to be a radiator) at design conditions.
  • If the flow temperature required is less than 55ºC for all emitters, no additional measures are needed for the refit.
  • If the flow temperature is higher than 55ºC in some of the emitters, those emitters must be replaced by larger surface heat exchangers.
  • If the flow temperature is greater than 65ºC for all emitters and it is not possible or not desired to carry out replacement, a high temperature heat pump will need to be used.

Benefits will be made if the heating capacity can be reduced by ensuring that air infiltration losses are reduced, insulation to the building is improved and glazing is upgraded, all of this in line with the latest Building Regulations. The result of this is not only to reduce the heating requirement, but also to lower the water temperature needed for heating.

Accurate selection of an air source heat pump (ASHP) to meet the demand at a range of outdoor air temperatures is absolutely vital. Remember that ASHP output is a function of heat source temperature and water flow temperature, so both efficiency and kW rating will decrease at colder times of the year. The capacity of the heat pump needs to be matched to the energy demand during cold periods. The following example illustrates the factors that need to be considered. Figure 2 shows the heating demand as 11 kW with the red line, the room design temperature is 21ºC and the ambient air design temperature is -3ºC (green). The diagonal purple line intersects the heat pump output curve to give the bivalent or balance point (1ºC ambient). From Figure 3, the heat pump contributes 100% of the heating demand at this balance point.

Fig. 3: Selection procedure, step 2

When the ambient temperature falls below the balance point, Figure 4 shows how the heat demand increases, while the heat pump output falls, such that about 3 kW of supplementary heating has to be provided. Other factors that may affect the heat pump output are:

  • Wind speed affecting fan on outdoor unit
  • Positioning in relation to the building (dead areas)
  • Orientation and shading
  • Geographical height and area
  • Defrost requirements

Hot water storage

The ability to provide sufficiently high water temperatures for stored hot water is also important, both from an efficiency point of view and also to meet Health and Safety legislation aimed at preventing legionella. For heat pumps specified primarily for hot water production, high temperature models are able to achieve stored hot water temperatures of up to 65ºC without the need for supplementary heating. These typically use refrigerants such as R290 (Propane) or R134a to achieve higher temperatures within the vapour compression cycle. Lower temperature heat pumps will require support from a supplementary heat source to reach the maximum temperature – this might be from another heat source, such as a fossil fuel boiler, or from a boost electric immersion heater.

Correct selection of the hot water cylinder with an appropriately sized heat exchanger to ensure maximum heat transfer is important as heat pump systems usually require a larger heat exchanger surface area. This is particularly important for air source heat pumps to ensure that stored hot water temperatures can be maximised in the summer months when the heat pump output will increase due to higher ambient air temperatures. Cylinder specification for air source heat pumps therefore needs to becarefully considered in line with performancecharacteristics of the heat pump.

Commercial cylinders compatible with heat pumps are available in sizes up to 4,000 litres, with coil size bespoke designed to exact specification. Hybrid systems of solar thermal and heat pumps are also possible and becoming increasingly popular. Intelligent heat pump controls can optimise use of the solar energy before bringing on the heat pump compressor. The use of waste heat is also an interesting application for heat pumps. Heat pumps installed at the first supermarket in Ireland built to Passivhaus standards use the waste heat from the chiller cabinets at around 30ºC, which is circulated directly through the system and used to provide hot water at 60ºC for the staff canteen and washrooms, and customer toilets.

Fig. 4: Selection procedure, step 3

In the current economic climate, many businesses will find the costs of a new renewable heating system off-putting. But building managers should bear in mind that the Renewable Heat Incentive(RHI) comes into effect in 2011, which will provide a direct financial return for every kWh of renewable heat generated. And to minimise the impact of the initial capital costs of installation, leasing schemes can cover the complete project outlay and spread the costs over up to 15 years. Repayments can very often be met by the savings made on fuel bills, and payments from the RHI will also make a useful contribution.

Fig. 5: CO2 Emissions and lifetime costs for various fuels

Figure 5 measures total lifetime costs against different fuels and also total CO2 emissions (electricity based on 0.422kgCO2 / kWh) for a three-bedroomed new-build semi-detached house over a 20-year period. Heat pumps are a viable option in commercial buildings, for hot water as well as space heating, offering great flexibility over installation and system design. It is a question of specifying the right heat pump for the specific task in mind, and siting it to gain the optimum performance.

© Terry Welch

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