CPD programme

PD PROGRAMME Professional development integrating gas-fired and electrical heat pumps with heating and hot-water systems This CPD explores how systems using air source heat pumps are evolving The CIBSE Journal CPD Programme This CPD will consider the integration of (electrically powered) vapour compression and (gas powered) gas absorption air source heat pumps in non-domestic applications. With the recent moderation1 in projected natural gas price increases over the next 15 years, a strong economic case for using natural gas in areas where it is available for heating and hot water services is likely to continue for the foreseeable future. However, even in existing installations the inclusion of a low carbon or renewable technology such as heat pumps can provide an attractive supplemental, or replacement, heat source. The financial benefit of these alternative methods is magnified by the tax and grant incentives provided by schemes such as the UKs Renewable Heat Incentive (RHI)2, which makes significant payments for heat sourced from air source heat pumps that meet appropriate quality standards. The operational characteristics for heat pumps are such that, as the difference in temperature between the source (the outdoor ambient air) and the output been developed to allow the injection of refrigerant part way through the compression process. This is shown in Figure 1, where a small amount of economising refrigerant is initially separated off (as a liquid), after it leaves the condenser, passed through an expansion device thereby reducing its temperature and pressure and then used to cool the main, high-pressure refrigerant flow, across a heat exchanger. The superheated but cool, medium pressure economising refrigerant is then injected into an intermediate point in the compression process. This economised vapour injection (EVI) arrangement Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers. Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a longterm commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK). CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSEs requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institutions guidance. Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at www.cibsejournal.com/cpd while the information they contain remains current. You can also complete the questionnaire online, and receive your results by return email. cPd QUesTionnAire To take part in this months cpd, simply read through the module here, then follow the link at the bottom of this section. alternatively, visit www.cibsejournal.com/cpd you will receive notification by email of successful completion, which can then be used to validate your cpd records. of a natural-gas-fuelled condensing boiler over the traditional vapour compression cycle heat pump. However, technology that was formerly only applied to larger commercial, multi-stage compressors (known as intercooling or economising) is now available in smaller compressors in particular, scroll types that have (the heated water) gets higher, the COPH the ratio of useful heat output divided by supplied energy will reduce. This will lower the output of the heat pump, so, at times of a buildings peak heating requirement, when it is coolest outdoors, the heat pump output will be at its lowest, so may require additional or alternative heating from another source, such as a condensing gas boiler. The carbon benefit for the inclusion of such technology is not always clear-cut, as it is open to the vagaries of the energy supply (electricity and gas) and the carbon embodied in the equipment. So to model true life-cycle comparative environmental impact requires local, site-specific knowledge, as well as assumptions about the production and transportation of the equipment. effectively splits the compression process into two stages within a single compressor. It acts to cool the bulk of the refrigerant in the compression process and so reduces the work of compression. As illustrated in the pressure-enthalpy diagram of Figure 1, EVI shifts the compression process to the left, and the refrigerant superheat is reduced at the outlet of the compressor. EVI is particularly beneficial at lower evaporating temperatures that is, at low outdoor air temperatures around 0C or below. It also increases the refrigeration effect, the useful amount of heat that the refrigerant can gain from the outside air. Pressure Condenser Control valve provides heat to load (eg, hot water) Condenser Economising vapour injected into compression Expansion device Expansion device Economising vapour line Counterflow heat exchanger Application of modern air source heat pump (AshP) technology Until recently, for a typical northern European cold day, the carbon impact and energy cost was likely to favour the use Economising vapour through heat exchanger Compressor Compression process Evaporator Expansion device Evaporator gains heat from air Expansion device Enthalpy Figure 1: Simplified functional schematic and P-h process diagram of air source heat pump employing EVI High pressure, high temperature refrigerant vapour Condenser Provides heat to load (ie, hot water) strong ammonia solution absorbs heat in the evaporator from the outdoor air so evaporating the ammonia (the refrigerant), which is then redissolved into the water (the absorbent) in the absorber. The pump then increases its pressure, ready to repeat the cycle in the generator. Effectively, this is the same operating principle as ASHP, but the electricallydriven compressor is replaced by the absorber/generator combination, powered by heat from gas combustion. This can be packaged into a low-noise unit that will include appropriate controls and ancillary components such as that shown in Figure 3. Compared with a typical vapour compression ASHP, absorption heating COP and output will vary much less with the outdoor temperature.4 As outdoor temperature falls from 5C to -5C, the reduction in capacity for an absorption system is less than 10%, compared with more than 30% for a similarly sized electric heat pump. This means that a properly sized GAHP may well be able to provide a consistent source of hot water throughout the seasons. For a GAHP, the COPH is likely to be 1.3 to 1.54,and a typical seasonal COP will be 1.4.5 This is 40% better than a typical gasfired condensing boiler. A recent study6 Generator heated by gas burner Heat exchanger Pump Strong solution Weak solution Expansion valve Throttling valve Absorber Evaporator gains heat from air Figure 2: The simplified process of a GAHP In applications which have little or no available natural gas, but do have gridsupplied electricity, there is an advantage in operating a modern ASHP even in more adverse conditions because it will out-perform simple electrical resistance heating. This is also very likely to be more carbon efficient than using other fuels, such as oil and coal. (The excellent research paper3 by Huang and Hewitt reports on the practical implementation of EVI, as well as indicating that such systems can be successfully and economically applied at temperatures below -1C.) gas absorption heat pumps (gAhP) As shown in Figure 2, a GAHP heats an ammonia and water solution with a natural gas burner (in the generator), and the high-pressure vapour is then passed to the condenser, providing heat for the load for example, hot water. The remaining liquid from the generator (weak ammonia solution) is passed via the heat exchanger into the absorber. The main flow from the condenser is passed through an expansion valve, and the now low-pressure, low-temperature, Figure 3: Commercially available GAHP (Source: Lochinvar) When sensing a demand, the ASHP is switched on while concurrently activating the pump between the heat exchanger and the cylinder. The cylinder immersion heater will be activated if the unit cannot provide domestic hot water (DHW) at suitable temperatures that is, higher than 60C. The cylinder temperature sensor provides the signal to activate the shunt pump and immersion heater backup, and even the secondary pump as required. The unit also has a standard anti-legionella programme to pasteurise the vessel. Such an application operating with ASHP flow/return temperatures of 63C/57C will work with a COPH of around 2.4 (dependent on model). This application of the ASHP has integrated controls to provide weather compensation in heating mode. So, for example, when applied to smaller systems it can provide heat for DHW with a small indirect cylinder (via a three-port valve), and also be used for underfloor heating or oversized standard radiators working on lower temperatures. When supplying DHW, it can operate at 63C/57C and then working solely in heating mode at 50C/45C. If natural gas is available, it may be better at lower external temperatures to indicated that GAHP were particularly successful when applied as part of a multivalent system. examples of AshP integration Figure 4 shows an example of a manufacturers application of an electrical air sourced heat pump providing heating for a storage hot-water vessel. ASHP units typically operate with a flow/return temperature difference of approximately 5K, whereas traditional boiler systems worked on 10K, and modern condensing boilers 20K. This means, when sizing an indirect coil (for ASHP), the coil size has to be larger to meet a particular load. Experience shows that it is difficult, practically, to source appropriately sized indirect cylinders, so a heat exchanger with an effectiveness of around 95% is used, as shown in Figure 4. The plate heat exchanger thermally connects the ASHP to the buffer vessel effectively making it part of a primary circuit. The capacity of that buffer should be designed to prevent undue cycling of the ASHP. This example system comes as a package that can work as a stand-alone water heater, with all controls required built into the ASHP including the cylinder sensor and a manually reset overheat thermostat. use a condensing gas water heater, with the ASHP pre-heating the incoming water in a similar way to the GAHP example below. The example illustrated in Figure 5 has a gas absorption heat pump integrated with a gas condensing boiler, providing lowtemperature hot water used, for example, for underfloor heating or low-temperature radiators and also pre-heated hot water to a gas fired condensing water heater. The thermal store, as in Figure 6, acts as a large, low-resistance header which can also integrate heat from other sources, such as solar thermal panels, using the additional coil in the cylinder. This allows the heat pump to continue working in its most efficient mode, while providing useful input to the hot-water system from multimodal sources. The thermal store also prevents legionella risk, as it is not being used to accumulate domestic hot water. When operating air sourced heat pumps at low external temperatures (approaching 0C), frost is likely to accumulate on the ambient evaporator coil, as the moisture in the outdoor air freezes, obstructing the coil. For ASHP, a typical solution is to run the heat pump in a reverse cycle by altering the direction of flow into, and out of, the compressor thereby using heat from the compressor and the load coil to Legend for Figures 4 and 6 TPRV IV Figure 4: Air source heat pump providing primary heating for domestic hot water (Source: Lochinvar) Temperature and pressure-relief valve EV Expansion-relief valve EXV TD Tundish NRV Non-return valve IV Isolation valve LSV Lock-shield valve PMP Pump HL High limit stat TP Temperature and pressure gauge PRV Pressure-reducing valve AVC Anti-vibration coupling AVM Anti-vibration mounts SFG System fill group FSW Flow switch AS Air separator DS Dirt separator DV Drain valve FR Flow regulator 3PV Three-port valve AV Air vent PHEX Plate heat exchanger TS Temperature sensor TS1 Heat pump return sensor TS2 Heat pump flow sensor BTI User circuit sensor BTS DHW circuit sensor STR In-line strainer TPRV Pump live Expansion vessel AV OUT TS2 AVC EXV FSW EV ASHP TS1 AVM PMP DV TP AS AVC DS IV TP Relay IV IV PHEX Cold feed AVM HL PMP TD Immersion control BTS Boost immersion heater EXV IV PRV Direct vessel IV IV FR SFG IV IN IV Building flow LSV EV TD NRV PMP Building return IV IV NRV Boiler Power to bypass Heating load valves via high limit RB200 EV EXV DV DDC Controller TD LSV NRV IV IV GAHP IV AVC EV AS IV PMP IV AAV HL TS TP IV IV DS TD AVC TP PMP IV PMP modulation cable TS IV PMP live by others NRV Control by others IV IV PMP Building return Building flow IV TPRV TD EXV EV IV LSV Pre-heat Water TD flow heater EXV EXV EV LSV IV EV TD NRV PRV Cold feed LSV TD TPRV TD Figure 5: A GAHP combined with a condensing boiler and a thermal store to provide pre-heating for dedicated domestic hot-water heater and heating for low-temperature heating circuit (Source: Lochinvar) provide a defrost for the outdoor coil; or by supplying hot gas directly from the outlet of the compressor to the inlet of the evaporator, so bypassing the condenser. During this period, there will be no heating provided to the load. In GAHP, the heat pump has an automatic defrosting system that also enables a continuous, but reduced, supply of heat to the load. With both types of heat pumps, this reduces the systems overall efficiency and, therefore, will affect the seasonal performance. Thermal stores (buffer vessels) need to be sized to prevent cycling of the air sourced heat pump, but also to provide a heat store for when it is in defrost mode. The size of the store needs to take into account various factors including: The output rating of the heat pump Whether there are backup gas boilers The number of compressors within the heat pump. A typical rule of thumb is to size the store at 20 litres/kW output of the heat pump, plus an additional 10 litres/kW output of any modulating boiler. The manufacturer of the equipment should be able to provide explicit guidance pertinent to a particular application. Tim Dwyer, 2015. RefeRenceS: With thanks to Steve Addis, of Lochinvar, for sharing his practical experience of gas-fired and renewable hot water production. 6 Busato, F et al, Two years of recorded 1 DECC Fossil Fuel Price Projections, Department of Energy & Climate Change, September 2014. 2 Non-Domestic Renewable Heat Incentive (RHI), accessed 25 May 2015. 3 Hewitt, N et al, The experimental analysis of the effect of ambient factors on the defrosting of economised vapour injection compressor ASHP in marine climates, International Journal of Refrigeration, Vol 36, Iss 3, May 2013. 4 Dieckmann J, Zogg R et al, Heat-Only, Heat-Activated Heat Pumps, ASHRAE Journal, January 2005. 5 Robur Technical Information, Robur (2008). data for a multisource heat pump system: a performance analysis, Applied Thermal Engineering, 2013 57. Module 79 July 2015 Fill in this months questionnaire online. You will receive notification by email of successful completion, which can then be used to validate your CPD records in accordance with your institutions guidance. Figure 6: A thermal store (buffer tank) with tappings for multiple direct heat sources and coils for indirect sources and secondary circuits (Source: Lochinvar) CLICK HERE TO FILL IN THE QUESTIONNAIRE