Heat pump science – simplified!

Explaining the physics and engineering behind heat pumps

Watch our video to understand how a heat pump works, and how we can use them in our parks and green spaces to harvest heat. Or read on to learn more.

What is a heat pump?

There are two ways you can warm up a cold room. One is to create heat, as you would by running a traditional electric heater. The other is to move heat into the room from elsewhere, and the most efficient way to do that is by using a heat pump.

Understanding how a heat pump works means remembering two key things from your school physics lessons. The first thing is that pressure affects the boiling point of a liquid. Lowering the pressure means a fluid will turn into a gas at cooler temperatures, while raising the pressure means the fluid must be hotter before it can boil. That’s why it’s easier to boil water on top of a mountain, where air pressure is low, than it is at sea level.

The second thing is that a gas turning into a liquid will release heat energy and warm the environment, while a liquid turning into a gas does the opposite - sucking in heat energy and cooling the environment. (This is why sweating cools us down, as the liquid sweat evaporates). 

The combination of these two effects means that by controlling the pressure, we can make a liquid turn into a gas, or a gas to a liquid, whenever we want - warming or cooling the environment in the process. A real-world heat pump is essentially a loop of sealed tube where this process can take place. At different points in the circuit there are interfaces with pipes that deliver energy to the place you want to change the temperature of (ie the place you want to heat up or cool down), and pipes that bring energy from the outside – that’s where energy enters and leaves the central loop. The central tube is filled with a refrigerant, which can be any liquid that has the right blend of thermodynamic properties. Those properties will differ depending on the task, but commonly used refrigerants include ammonia, carbon dioxide, isobutane and hydrofluorocarbons.

The refrigerant is circulated around the tube. It enters the compressor in a gaseous state at a lower pressure and temperature. The compressor increases the pressure of the gas. It then goes into the hot side of the tube where, thanks to the high pressure, it turns into a liquid, releasing heat energy. In the example shown here, this released heat goes into the ‘heating distribution system’ – in other words, the radiators and hot water tanks inside the building. The refrigerant is then pushed through an expansion valve, which dramatically lowers the pressure. This drop in pressure begins the process of converting the refrigerant back into a gas, which is completed in the cool side of the tube. The energy it needs to do this is pulled out of the surrounding environment, such as the ground beneath a park, the water in a lake or the air. In our example, this heat comes from the ‘ground array’ of pipes laid beneath the top layer of earth. The cool, low pressure refrigerant then passes back into the compressor, and the cycle starts again.

The cool side can be fairly cool and still contain plenty of ‘heat’ for the heat pump to transfer. When in liquid form, the refrigerant is so cold that it absorbs heat from the pipework it is in contact with - even when the fluid in those pipes is below zero degrees! The ground itself, or the water of a lake or river, might itself only be a few degrees above freezing, but still offers a source of energy. By pumping the refrigerant round the system, the not-very-hot heat from the external environment can be converted into usable heat.

There are lots of ways this technology can be used. In a fridge, or air conditioning unit, heat is moved from inside to the outside. In a heating system, the reverse is true and heat energy is moved from outside to inside. It’s possible to build a system that’s reversible - releasing heat to the outdoors in the summer and capturing it in the winter. 

Heat pumps do not involve combustion and so do not produce CO2 - or any other pollutants that threaten air quality – on site. Of course, the carbon emissions and air pollution associated with the electricity used by a heat pump depend on how that electricity was generated. If the electricity is zero-carbon, the heat supplied can also be said to be fully zero-carbon.

Heat pumps can also work in reverse – moving heat out of warm buildings and transferring it to a natural ‘sink’ which can be the air, the ground or a body of water. This can help keep buildings cool in the summer.

Either way, harvesting heat energy from the external environment, rather than creating it in place, makes heat pumps one of the most efficient forms of heating available. Putting just one unit of electrical energy in will produce up to five units of heat energy. It’s basically magic.

Coefficient of performance

A heat pump’s coefficient of performance (abbreviated to COP) is the ratio of heat output to electricity input. Real-world ground and water source heat pumps typically achieve a COP of between 2 and 6. Good heat pump designs achieve better COPs.

The coefficient of performance is typically better with a warmer heat source (air, ground or water body) and a lower temperature requirement on the demand (building) side. This means that the COP is normally higher in the shoulder seasons of spring and autumn, and on milder winter days, and lower when the weather is coldest. Because COP fluctuates over the course of a year, the costs and benefits of heat pumps are calculated using an average figure called the seasonal performance factor (SPF). SPF also takes any ‘parasitic’ electricity consumption such as circulation pumps into account so that the system’s true running costs can be known. Real-world ground and water source heat pumps typically achieve SPFs of between 3 and 5.

The variation of COP with supply temperature (where the heat is required) has implications for the design of building heating systems. Underfloor heating systems and special radiators can operate at lower temperatures than conventional radiators, enabling heat pumps to operate more efficiently.

Building blocks

The refrigerant that moves around the cycle of evaporation-compression-condensation-depressurisation – and the machine parts that make that cycle happen - are contained within the heat pump appliance or plant unit. The heat collector and heat use sections are normally simple fluid circulation systems consisting of pipes and pumps.

The COP only refers to the heat pump appliance/plant, but the SPF considers the whole system.

Heat pumps in the real world

Heat pump appliances/plant

Whatever the type of heat collector used (ground- or water-source, horizontal or vertical, closed or open loop), the heat pump appliances or plant units follow very similar designs. Sometimes the same model can be used for a ground- or water-source system. 

For information about the UK and European markets for commercial-scale ground- and water-source heat pumps, see module 5.6.

Size and appearance

Often the heat pump unit is housed inside a single package that can range from the size of a large domestic fridge-freezer, to a box or uncontainerised (skid-mounted) assembly several metres in length. Some smaller units are shaped like a tall fridge-freezer – minimising the floor space needed – whereas others are closer to a cube shape or are longer than they are tall. The volume of the heat pump package or skid does not increase proportionally to the capacity of the machine; a 200kW capacity unit is unlikely to require twice as much space as a 100kW unit.

Product weights range from around 100kg to several tonnes.

Heat pump units are normally offered as standard models within a manufacturer’s range, but units may be produced to bespoke specifications in the case of large installations. For medium-to-large schemes, it is common to specify more than one heat pump unit which will operate in parallel. Many models’ visual designs give the effect of sleek, unobtrusive metallic boxes. 

Lifetime

The lifetime of a regularly-serviced ground or water source heat pump unit is typically at least 20 years and commonly 25 years. This compares favourably with combustion boilers (e.g. gas or oil) that typically last 8 – 15 years.

Specifications

Most heat pump models are designed to produce a maximum outlet temperature (on the heat user side) of between 55°C and 65°C. Many models can achieve higher temperatures on a temporary basis, allowing them to periodically heat domestic hot water tanks above the threshold for controlling the growth of Legionella bacteria. The outlet temperature has important implications for the compatibility of the heat pump system with its end use. Module 5.1 explores these implications where heat pumps are used to provide space heating.

Models capable of delivering higher output temperatures on a continuous basis are available. These typically do not achieve such high efficiencies as lower-temperature models, but may be necessary to be compatible with the heat use in question.

Noise emissions can vary substantially between models, and vary with the machine’s output level. Ground and water source heat pumps are typically louder than the quietest available gas boilers, but quieter than the noisiest gas boilers (particularly older models). By siting the heat pump unit suitably and installing noise attenuation measures where appropriate, the noise emissions from ground and water source heat pumps will be acceptable in most contexts. Note that noise is a more common problem for air source heat pumps, the outdoor parts of which can be difficult to acoustically insulate without compromising the system’s performance – so selection of a quiet model is often of high importance.

A heat pump unit’s maximum power consumption may be high compared to a building’s other electrical loads, so a higher-capacity power supply may be required. Larger heat pump units tend to use three-phase power.

Location considerations

Heat pump appliances/plants are usually located in the same kind of places as combustion boilers: utility rooms, plant rooms or dedicated cupboards. If space, accessibility or noise constraints mean that a heat pump unit cannot be sited within a building, there is the option to house it and ancillary equipment such as tanks and pumps in a separate weatherproof enclosure.

Ground source heat collectors

The majority of ground source heat pumps employ one of three designs for their heat collectors:

1. Pipes buried in trenches, laid down as horizontal coils;

2. Straight pipes inserted into deep vertical boreholes;

3. ‘Open loop’ systems that extract underground water into pipes, pass it through the heat extraction system, and return it to a different underground location.

The first two types of system – ‘closed loop’ systems – are more common in the UK, and can be deployed in a wider range of locations. Open loop systems can be superior in places where groundwater conditions are favourable, but these installations require additional environmental permits and specialist design. This toolkit focuses on closed loop systems – but includes reminders to discuss the potential for open loop systems with specialist companies at appropriate points in the project development lifecycle.

Vertical ground loop systems employ boreholes which can range from a few tens of metres deep to several hundred metres. In the UK, permitting rules and the relative lack of high temperature geothermal resources (other than in specific locations of the country such as Aberdeenshire and Cornwall), mean that boreholes of less than 200 metres depth are favoured. The local geological conditions will be an important factor in determining the borehole design depth. Borehole drilling companies provide services for lots of different types of project, not just ground source heat projects – and have access to different drilling rigs to suit particular requirements.

In some locations, only one ground loop type may be feasible. For opportunities where either system could be deployed, the trade-off between horizontal and vertical ground loops must be considered. 

For details of the types of locations where each type of ground heat collector is suitable, see module 5.2.

Lifetime

The buried pipes are made of a durable, chemically resistant polymer material that has a life expectancy of at least 50 years, with many projects designed on the basis that lifespan will exceed 100 years. The design life of closed loop ground source heat collectors is therefore comparable to that of gas and water main infrastructure.

Appearance

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Once buried, neither horizontal nor vertical ground heat collectors are visible. One or several manhole covers may be specified to provide access to the manifold chambers where pipes are connected together.

Water source heat collectors

The bodies of water from which heat can be extracted include ‘surface’ water – rivers, lakes, the sea – but also underground water bodies, typically flooded mines. Surface water in rivers, lakes and reservoirs covers the most relevant sources for park heat opportunities. Marine water source heat pumps could be appropriate in coastal green spaces, but their general inapplicability means that they are not covered in this toolkit. Underground mine water is also a very localised resource which often presents particular difficulties in terms of estimating the quantity of heat that can be accessed. For parks and green spaces located above old mine workings, specialist advice can be sought to establish whether a minewater heat project is worth pursuing. Minewater heat pump opportunities are not the subject of this toolkit due to their non-general applicability.

As with ground source systems, water source systems can also be closed loop or open loop. Closed loop systems involve installing either coiled pipes fixed to a frame (“pond mats”) or a plate heat exchanger in the river, lake or reservoir. The installation is either fixed to the bottom of the water body, or a floating structure is anchored such that it is suspended 1 to 2 metres below the surface. Open loop systems extract water, normally through a filter, and transport it to the heat pump or an intermediate heat transfer circuit. The ‘spent’ water is then returned to the river downstream of the intake, or returned to the lake some distance away from the intake.

Lifetime

Closed loop water source heat collectors that use coiled pipes use the same durable, chemically resistant and UV-resistant polymer materials as ground source heat collectors. The design life of this type of heat collector is normally 50 years.

Appearance

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Once submerged, the heat collector equipment is not visible. The visibility of the pipes that come ashore can be addressed in the design of the system where necessary.

The economics of heat pumps

Simple cost of heating

Considering the ratio between the amount of useful heat generated, and the amount of energy that is put in, heat pumps achieve very high efficiencies:

The difference between the unit cost of the input energy and the price paid for that input energy determines the efficiency (i.e. if the price for 40 units of gas costs £0.85, but you pay £1 for the input energy, then the gas boiler is 85% efficient). Dividing the cost of the input energy by the price of the input energy gives the effective cost of the heat supplied. 

A water source heat pump is considered to be 400% efficient, because for every 1 unit of electrical energy input, you will get 4 units of thermal energy. Comparing a ground source heat pump (340%) to a natural gas boiler (85%), the heat pump is 4 times more efficient. As long as the electricity price is less than four times more than the gas price, the heat pump will be more cost effective.  

Fuel and electricity prices vary enormously depending on factors that include the total purchasing power and procurement strategy of the customer, annual energy consumption at a particular site and geographical location – and of course they vary over time. This means that simplistic financial appraisals can show different technologies giving the best financial performance at different sites. Therefore, it is recommended that site specific investigation and financial modelling is undertaken.

It must be remembered that the total cost of electricity or fuel consumption must include all relevant elements of the bill and not just the basic unit price. Climate Change Levy, Distribution Use of System (DUoS) and other per-unit charges should be included in the cost of heat calculation.

Subsidies

The current subsidies available for heat pumps are covered in the dedicated “Subsidies” Module, with “Markets and Funding” in a separate module. 

Economies of scale

Heat pumps and the associated infrastructure will not always display economies of scale. Although the cost per kW capacity does tend to fall between domestic-scale and medium commercial scale (up to around 300kW), larger machines are more likely to be bespoke rather than off-the-shelf and so economies of scale may stall or be reversed at the megawatt scale.

Ground and water source heat collectors do display strong economies of scale, and since the heat collectors typically represent between ⅓ and ⅔ of the total capital cost, installations as a whole can be expected to show falling costs per kW as capacity increases.

Lifecycle costs

The simple cost of heat measure – plus a per-unit subsidy, if it’s available - gives us a broad indication of whether a park heat opportunity has the potential to make financial sense and be affordable in the long run. 

To assess whether an investment in a park heat scheme stacks up against the alternatives, the lifecycle costs and benefits must be considered, alongside the cost of capital and adjustments for the time value of money. 

In addition to energy costs and subsidies, lifecycle costs and benefits also need to include capital costs, non-energy operating costs, other sources of revenue, plant replacement costs and any impacts on an organisation’s liability for taxes and levies. Lifecycle costs and benefits consider the lifespan of different elements of the system.

Some important considerations for analysing lifecycle costs include:

• Heat pumps last 2-3 times longer than fossil fuel boilers (and ground and water source heat collectors last 2-3 times longer again);

• Grants or lower-cost loans may be available for low-carbon heat projects;

• Selling heat and claiming low-carbon subsidies may create new administrative tasks for the operator of the heat pump scheme, potentially with budgetary implications;

• Although normally broadly comparable, maintenance and servicing costs will be different for heat pumps and fossil fuel boilers; electrical heaters have no such costs.

At present, assessments of the financial viability of ground- and water-source heat pump schemes do not always acknowledge the differing lifetimes of different parts of the system. If the assessment protocol allows, the lifespans of heat pump plant and heat collectors should be modelled separately (around 20 and >50 years, respectively). Potential investors are not always used to placing value on long-term infrastructure other than buildings, so park heat opportunities may need to be presented in different ways to effectively communicate the total costs and benefits of ownership versus the alternatives. Fossil fuel heating systems, where the long-term infrastructure (e.g. gas network, oil production and supply chains) is owned by third parties, are not subject to such considerations.

Financial assessment

At the early stage, which is the focus of this toolkit, the financial performance of greenspace heat schemes is most efficiently assessed by calculating the simple payback. This means calculating the net financial benefits (savings and revenues minus costs), and dividing the estimated capital cost by this number. The result is the number of years that it would take to repay the original investment, taking a simplistic view that the value of future cash flows is the same as current cash flows.

In subsequent stages of a project’s development, it will be appropriate to calculate more complicated but more useful metrics of financial performance. Three of these metrics are the Net Present Value, the Levelised Cost of Heat and the Internal Rate of Return. All three require a project cashflow forecast to be prepared, modelling future costs and revenues on at least a yearly basis.

Net Present Value

The Net Present Value is the amount of money that could be offered to an investor today that would be equal to the overall value of the proposed scheme over the course of its lifetime. The calculation of the NPV involves a valuation of the costs and benefits that the scheme will deliver in the future, and discounting them by a certain amount in recognition of the fundamental economic concept that future money is worth less than present money. The more distant a cash flow is in the future, the more heavily it is discounted.

A high number for the NPV means an attractive investment.

Levelised Cost of Heat

The Levelised Cost of Heat (LCOH) is simply the Net Present Value of the total cost of building and operating the heat pump scheme over a period of time, divided by the total amount of heat generated over that period. The period over which these values are assessed could be the lifetime of the heat pump (ideally factoring in a residual value for the heat collector, which will not have reached the end of its life), or the lifetime of the heat collector (with heat pump replacement costs having been factored in to the calculation).

Internal Rate of Return

The Internal Rate of Return is a less-intuitive concept, but an equally useful measure of the financial attractiveness of an investment. It is the ‘discount rate’ at which the Net Present Value of the investment equals zero. IRR is expressed as a percentage, and assessors are looking for an IRR that is higher than their cost of capital in order to consider the investment to be financially worthwhile.

Basic configurations of green space heat schemes

Supply to a single user (‘standalone’ installations)

Heat collector is connected to a single heat pump, which supplies heat to a single building. The heat collector and building may be close together or distant. It is not classed as a heat network.

Heat networks with centralised generation

At least one heat collector is connected to a heat pump or co-located heat pumps. The heat pumps’ location is often referred to as the Energy Centre, but does not need to be in a dedicated building. The heat pumps then supply heating to several heat-user buildings by circulating hot water through the ‘network’ and eventually transferring heat to the buildings’ internal systems. Technically, a heat network can have as few as two heat user connections. 

In this configuration, the pipes of the network need to be insulated in order to reduce the in-transit heat loss. Fewer, larger heat pumps can be installed than would be the case if each building required its own heat pump. Larger heat pumps may achieve better efficiency and offer economies of scale in terms of capital and operating costs relative to smaller heat pumps.

‘Shared Ground Loop’ heat networks

At least one heat collector is connected to a distribution network that connects directly to heat-using buildings (they share the ground loop). The fluid in the network circulates at a low temperature - close to the temperature of the ground - and so the pipes do not need to be insulated because there is no heat loss. Each connected building houses its own heat pump which supplies its own needs. There is not normally an ‘Energy Centre’, so this configuration is sometimes termed a decentralised heat network. Again, the term heat network applies when there are at least two heat user connections.

The reduced cost of distribution pipework (no insulation) is the primary benefit of a Shared Ground Loop heat network, and can offset the additional cost (relative to a conventional heat network with centralised generation) of installing a larger number of smaller heat pumps.

Models to link supply and demand

Greenspace Scotland has developed a simple framework for describing the different models through which park-generated renewable energy – including ground- and water-source heat – can be used. Their 3 main “designs” comprise:

1. Island model – renewable energy is generated and supplied directly to an end-use within the green space. For heat pump schemes, this will normally mean that one or more buildings (or other users of heat) within the park are served by the ground- or water-source installation:

   1. ‘Island’ standalone installation(s) – each building is supplied from its own heat collector

   2. ‘Island’ heat network – two or more buildings are supplied from a shared network that connects them to one or more heat collectors

2. Generator model – renewable energy is generated within the green space and exported to a user or users beyond its boundaries. For heat pump schemes, this means connecting heat demands to a new heat network or connecting the heat pump installation to an existing heat network. Standalone schemes, where the ground-/water-source heat system supplies a single customer outside the park, are also possible. Heat is transported through the park, across the boundary and to the end user through pipes.

3. Host model – a green space hosts infrastructure that forms part of a wider local energy system that does not rely solely on greenspace-generated energy. This infrastructure hosting may be purely ‘passive’ – such as greenspace providing land for pipe routing – or may also involve locally-integrated energy generation within the park. 

For heat pump schemes, the boundary between ‘generator’ and ‘host’ models may be blurred. The opportunities that are the focus of this toolkit are classified as ‘island’ or ‘generator’ models only, with the integration of ground and water source heat installations into complex wider local energy systems considered to be a special case of the ‘generator’ model.

Footnotes:

1. Smaller systems can be considered to be appliances; larger systems are more commonly referred to as ‘plant’.

2.  For discussion of future land use restrictions in the context of parks and greenspace, see module 5.3.

3. Real-world performance – condensing boilers are theoretically capable of achieving efficiencies close to 100%, but this is rarely achieved in practice.

4. This calculation compares only the useful heat generated against the electricity consumed: the free and zero-carbon heat extracted from the ground or water is not counted.

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