Cares renewable ENERGY

Transcription

1 Cares renewable ENERGY Heat pumps Handbook

2 Renewable Energy Technologies This Handbook aims to help you to: Learn the fundamentals of Heat Pumps. Explore and discuss the range of technologies and options available to your community. Evaluate which technologies or options may be appropriate for your community. Identify and access valuable online resources for further information and advice. It discusses the variety of technologies that have been employed by community groups across Scotland. The principles of how the technology works is provided along with the key issues regarding installation and operation as well as environmental impacts. The Heat Pumps handbook includes the following sections: Technology description. Technologies available on the market. System requirements. Is the renewables technology suitable for your community group? Introduction to available schemes and grants. Tips for project development. Environmental aspects. Case study. This Handbook is intended as an introductory text, covering the main aspects and issues that need to be considered for each of the technologies listed above. A separate set of Toolkits, (available Autumn 2013) will provide more detailed guidance and tools to assist community groups and rural businesses to develop a renewable energy project. Like the Handbook the Toolkits will be available on the CARES web site: localenergyscotland.org.uk

3 Heat pumps Technology description Heat pumps extract low- grade 1 energy from the environment and convert it into usable energy at a higher temperature suitable for space and water heating. Heat pumps collect energy from three main sources: Ground. Air. Water. A heat pump is (normally) powered by mains electricity. The ratio of heat output to the amount of electricity input is known as the coefficient of performance (CoP). For example, if it takes 1 unit of electricity input to produce 3 units of heat output, then the system has a CoP of 3. Electricity has substantially higher cost and CO 2 emissions per kwh than other fuels, so systems powered by mains electricity need to have a CoP of at least 3 to provide savings in costs and CO 2 emissions when compared to using mains gas for heating. Another metric of performance is regarded as more accurate is the CoP over a full heating season. This is termed a Seasonal Performance Factor (SPF) and refers to the efficiency of the heat pump when operating over the heating season rather than at specified temperature test points. This metric provides a more accurate reflection of the heat pump running costs. Hence, although the heat source is renewable, a heat pump is only usually classified as renewable, if its performance ensures that net CO 2 emissions savings are made, comparing the heating fuel saved, vs. the mains electricity used. The EU Renewable Energy Directive uses SPF in the calculation of heat pump performance. Several factors affect the CoP, a crucial one is the temperature needed by the heating system. The CoP will be higher for heating systems that are designed to use lower temperature heat (i.e C). This includes underfloor heating systems, low temperature radiators and air heating systems. Hence heat pumps are particularly well suited to buildings that have these lower temperature heating systems. As these can be expensive to retrofit, new buildings already incorporate low temperature heating are a good application for heat pump technology. Heat pumps work by means of the vapour compression cycle. This is essentially the same technology used in a refrigerator but operating in reverse. The diagram in Fig 6 illustrates this for a ground source heat pump. 1 Low- grade energy is energy that is difficult to convert efficiently into other forms of energy (e.g. the thermal energy in the ground). High- grade energy is energy that is easy to convert efficiently into other forms of energy (e.g. electricity).

4 Figure 6: Heat pump vapour cycle The heat transfer fluid circulates through a collector loop passes through the source of heat (ground, air or water). The warmed heat transfer fluid is then pumped through an evaporating heat exchanger where the small increase in temperature is transferred to a refrigerant gas in the heat pump circuit. This warmed refrigerant gas is then compressed by means of an electrically driven compressor causes the gas to become hot. This hot gas is pumped around a condensing heat exchanger where the gas condenses to liquid with the release of heat is transferred to the distribution system. The liquid then passes through an expansion valve where it cools further and is pumped back through the evaporator heat exchanger to start the cycle again. Hence, a single heat pump comprises three separate loops. Available technologies Three main types of heat pump are used, using the three types of heat source (ground, air and water). Ground source heat pumps (GSHPs) The ground in the UK is heated to a temperature of approximately 12 C by the sun (this varies by latitude, exposure and soil conditions), so ground source heat pumps are actually making use of solar energy. There are two ways of extracting this heat, using two different types of collector loop: Horizontal ground loops. Vertical ground loops. Most ground source heat pump installations in the UK are closed loop systems. The heat transfer fluid (usually a water/ glycol mix) is pumped into the ground loop where it collects heat and then is returned to the heat exchanger. Here it is used to evaporate the refrigerant, which will not leave the heat pump enclosure. Open loop systems, by contrast, pump water out of the ground and then return cooler water to the ground.

5 Open loop systems require additional consents and are comparatively rare in the UK so this section will focus on closed loop systems. For further detail on the challenges and risks associated with closed and open loop GSHP installation, SEPA have published a guidance note 2. Horizontal ground loop 3 A horizontal ground loop uses a length of spiral coil/pipe, known as a slinky, laid in shallow trenches. Pipes are normally located approximately 1.5m below ground level to avoid the risk of frost. The rule of thumb is that 10m of slinky needs to be laid for each 1kWth of heat output, so a system with a maximum heat demand of 10kWth would require approximately 100m of trenching. As trenches need to be separated, this can require a considerable area of ground to be excavated. The main advantage of horizontal over borehole systems is the cheaper overall installation cost. Once installed, the ground can normally revert to its normal use (e.g. lawn, pasture, agriculture). Vertical ground loop A vertical ground loop is installed by drilling a bore hole which can be up to 100m deep. The ground loop is then inserted into this hole. Compared to horizontal loops, vertical ground loops have a much smaller space requirement for installation but good access is required to get the drilling rig onto the site. Unless data is available from geological survey in the immediate area, a test hole and/or a geologist survey is recommended prior to project commencement. Bore holes are a costly option and are usually only considered when available space for horizontal loops is restricted or if the soil conditions are inappropriate. Air source heat pumps (ASHPs) Air- source heat pumps (ASHPs) work on the same principle as ground- source heat pumps, taking low- grade thermal energy and converting it to useful heat by means of the vapour compression cycle. ASHPs are in common use in commercial- scale Heating, Ventilation and Air Conditioning (HVAC) systems as they can supply both heating and cooling. Where ASHPs provide space heating, they do so by one of two distribution methods: Air- to- air Air- to- water 2 b392-49b1- be b17d39f3&version=- 1 3 Examples of sinking a borehole (vertical loop) and a slinky cable (horizontal loop) for Ground source heat pumps. Source: Carnon Contracting and Andrew Engineering

6 Air- to- air heat pumps are common in HVAC systems where low temperature warm air is distributed via ducts. Air- to- water systems are used with low- temperature distribution systems (as with GSHPs) such as under- floor heating or low temperature radiators. Compared with GSHPs, ASHPs have the advantage of being cheaper to install, but the disadvantage that in cold weather the air (i.e. the source of heat) will be at its lowest temperature when space heating is needed most. The pump thus has to work harder to supply the heat required, so the efficiency of the system drops rapidly. However, if an ASHP is combined with another source of heating, the system can be programmed so that the pump only works when it would operate above a pre- set coefficient of performance. The overall efficiency of the heating system will thus have been improved. It is less common to use ASHPs to provide domestic hot water due to the higher temperature gap between the heat source (air) and need to keep hot water above 60 o C. This may require a supplementary form of heating to reach the necessary temperature levels. Water source heat pumps (WSHPs) Water- source heat pumps harvest their low- grade energy from nearby bodies of water, rivers and even abandoned coal mines. The heat can be collected, as in ground source systems, either by an open loop or a closed loop. Water abstraction from rivers or other public bodies of water for use in open- loop heat pumps would require permission from the Scottish Environmental Protection Agency, and coal mines the Coal Authority, in addition to any other permissions required. A key advantage of WSHPs is the thermal properties of water as a heat source, this enables WSHPs to operate at higher efficiencies (hence lower cost) than both GSHPs and ASHPs. Oxygen and contaminants in river water may be a concern in some circumstances as this can cause heat pump failure and in worst cases a refrigerant leak from a closed system. A loch could be a suitable heat source and should be considered as an option if adjacent to the heat demand. However, good proximity is key; and the acidity and impurities in the water need to be checked to ensure compatibility with the heat pump system. The water source should ideally be fairly close to the property, and should not require pumping up any significant height as the power used for pumping the collector loop needs to be taken into account in overall system performance. System requirements for heat pumps Heat pumps work best where the gap between the source temperature and the delivery temperature is at its lowest. Conventional radiators require circulation temperatures of between 55 C and 80 C, but underfloor heating or low- temperature radiators require temperatures of between 30 C and 40 C so heat pumps tend to be used in conjunction with these low- temperature distribution systems. Low- temperature distribution systems are less responsive than high- temperature systems there is a time lag, usually of several hours before the maximum heat output is reached so low- temperature systems work best in buildings that are maintained within a constant temperature band, examples would be care homes or timber drying kilns.

7 As heat pumps are normally combined with low- temperature heat distribution systems with slow response times, they are also best suited to highly efficient buildings with low heat loss. Draughty or poorly insulated buildings require a heat source with faster response times in order to maintain thermal comfort. In Scotland this means that traditional stone built building may be less suitable as the heat pump may struggle to match the energy losses from the building fabric. Buildings with poor thermal performance will have to be brought up to current regulatory standards to maximise the potential gains of a heat pump or to avoid increased costs and emissions from a poorly performing system. Are heat pumps suitable for my community group or rural business? Your community group or rural business could consider installing a heat pump if: The property is off the gas grid. There is an adequate heat source available (area of ground, potential for borehole, river or body of water, former coal mine). A low- temperature heat- distribution system such as under floor heating or low temperature radiators would be suitable for the property. The building is well insulated or can be brought up to modern standards of thermal performance cost- effectively and without technical risk. Further information is available by contacting Local Energy Scotland on Introduction to available schemes and grants In the UK, ground and water source heat pump systems for non- domestic properties are supported by the RHI mechanism, launched in This is not a grant scheme, but a long- term (20 year) revenue stream designed to encourage investment. Air source heat pumps are not currently supported under the RHI. However, air source heat pumps have been proposed to be included in RHI Phase II which is due to be launched during the first half of The RHI provides a payment per kwhth (kilowatt- hour thermal) of heat generated by a qualifying installation. The subsidy paid depends on the size of the installation and is slightly lower for systems over 100kWth. For comparison, a domestic heat pump is typically around 5kWth in size for a small (less than 100m 2 ) highly insulated property. Full details of the RHI are included in the Toolkit. Tips for project development It should be noted this is not an exhaustive list and all projects present individual circumstances to consider. 1. Planning permission is not normally required for closed loop ground and water source heat pumps. There are restrictions on ASHPs relating to noise and visual impact. There are additional restrictions in conservation areas. 2. Heat pumps are best suited to well- insulated buildings and installed in conjunction with low- temperature heat- distribution systems, rather than poorly insulated buildings with high- temperature radiator systems.

8 3. Heat pumps are most appropriate in buildings where a constant temperature is required as low- temperature distribution systems do not provide a fast response to changes in heat demand. 4. Large heat pumps (typically above 20kWth) are likely to require a three phase electrical supply. 5. Very large heat pumps may require upgrade of the connection to the local electricity distribution system. 6. Good practice for large GSHPs would include an environmental impact assessment. 7. Adequate metering of electricity input and heat output should be specified so that the performance of the system can be monitored, this will also align with RHI requirements. 8. The user guide provided by the installer should cover the complete system installed, not just the heat pump. This will ensure that the performance of the whole system is optimised. 9. Ensure the heat pump is appropriately sized, under sizing, whilst reducing initial outlay will result in inefficient performance and high running costs. Environmental aspects The carbon benefits from the use of heat pump systems is influenced by the type of heat source selected, the size of the system, the fuel displaced, the carbon intensity of electricity, and the efficiency of the distribution system. These same factors influence the energy cost savings from use of a heat pump. So maximising the cost savings will also maximise the carbon benefits. The main environmental issues include: Impacts on land during the installation of a ground loop or borehole. Noise from air source heat pumps, which are normally located outside the building. Leaks of the heat transfer fluid (ground and water based systems). Visual impacts from air source heat pumps, which are normally located outside the building. Impact on land Horizontal GSHPs disturb a substantial amount of soil and there may be associated transport impact in some cases. If land is contaminated then the impact is increases as is the risk of spreading pollution accidentally. The ecological impact depends on the quality of the work done to reinstate the land after the trenches are backfilled. GSHPs reduce the temperature of the ground and in poorly designed and installed systems this can lead to ground freezing. Vertical ground loops raise other issues especially where any aquifers are to be penetrated. There is a risk of damage to land and watercourses from refrigerant leaks and for water source heat pumps there is an impact from reducing the temperature of bodies of water or watercourses. Noise Heat pumps do make noise and ASHPs are louder than ground and water source heat pumps. The effect of noise from heat pumps will also depend on where the pump itself is situated and the sound transmission of adjacent structures. Good maintenance will prevent noise increase through worn equipment or faulty mounting.

9 Impact on Water Ground source and water source heat pumps can potentially have an impact on water resources, with the risk being higher for open loop systems. For this reason SEPA will need to approve abstraction of water from rivers or other water bodies, for mine water systems Coal Authority approval will be needed. Visual impact Ground and water source heat pumps have relatively low visual impact as the collector loops are hidden. ASHPs can have a very significant visual impact if the exterior air handling units are visible. Carbon impact The carbon impact of heat pumps depends upon the efficiency of the system (COP) and the type of fuel being displaced. The current mains carbon intensity of electricity is approximately three times that of gas per kwh so an average COP of at least 3 would be necessary over a heating season to generate any carbon savings when compared to gas. If the electricity powering the pump is renewably sourced then there will be a carbon saving in operation. If gas is not available and other fuels such as coal, oil or LPG are being displaced as a source of space heating then there will be a carbon saving in use. Savings are highest of all where electrical resistance heating is being displaced. Case studies Coalburn Miners Welfare Charitable Society provides the Coalburn One Stop Shop, which the hub of the community. They upgraded the heating of the function suites using six 14 kwth air to air source heat pumps. As part of a refurbishment project Ellon Church invested in a borehole, ground source heat pump providing 17 kwth. The church is utilised by a wide range of groups for a range of activities in the community, who now have access to a warm facility for them to enjoy.

10 Commissioned by the Scottish Government and Energy Saving Trust. Produced by Community Energy Scotland Limited and Ricardo- AEA Ltd Queen s Printer for Scotland 2009, 2010, 2011, 2012 This document was last updated July 2013