Heat pumps are widely seen as a key technology for the electrification of heating in buildings, reducing the electricity required to provide the same amount of heat by around three and a half times. Even if the electricity is generated using fossil fuels there can be efficiency gains, but the real aim is to enable renewable power to provide heat to buildings. A major challenge to increasing the share of renewable power in the grid is energy storage – and heat pumps can exasperate this issue. However, with larger heat pumps enabling longer-term energy storage, they can provide vital energy storage capability. Centralised heat pumps, with heat storage, may therefore be a key component of a net zero energy system.
HEAT PUMP EFFICIENCY
Heat pumps use electrical energy to pump heat ‘up hill’ from a cold area to a warmer area. Energy efficiency is a measurement of the useful energy output divided by the energy input. If we ignore the energy that is extracted from the cold environment, and divide the heat output by the electrical power input, a heat pump appears to give an efficiency of greater than 100%. This measurement is known as the coefficient of performance (CoP). CoP is typically between three and four, meaning the heat output is between three times and four times the electrical power input.
Even with fossil fuel power plants, heat pumps can give efficiency gains. Combined cycle gas turbine power plants achieve efficiencies of around 60%, while the combined efficiency of the electrical transmission and distribution system is around 90%. With a CoP of 3.5, this will still produce 1.9 times more heat then if the natural gas was burned in a central heating boiler.
Heat pumps can achieve higher CoP when the temperature difference is smaller. This means that trying to achieve the same temperature inside a building when it is very cold outside doesn’t just require more heat to be generated, but the efficiency of the heat pump can also reduce. This has a double impact on the energy demand. For example, if a building is to be heated to 20°C and it requires 40kW of heat to maintain this at the average outside temperature of 10°C, then with a heat pump operating at a CoP of four, it would require 10kW of electrical power.
The levelised cost of energy (LCOE) for wind and solar is already the lowest of any power source, and it continues to fall. Solar plants in the desert can produce electricity during the day for a fraction of the cost of fossil fuel plants. It’s not the economical generation of renewable power that’s preventing a rapid shift to a zero carbon energy system. The challenge is producing power close to the point of use and at the time that it is needed. Thermal power stations can be built close to cities and dispatch power whenever it is required. Renewable power makes economic sense when it represents a small share of the energy mix, and conventional power stations can step in when needed, but a fully renewable system is a very different proposition. Factor in the costs of long-distance transmission and long-term energy storage, and the actual cost of delivering ‘cheap’ renewable energy is no longer cost-competitive.
As an increasing proportion of our power is generated by wind and solar energy, the need for energy storage and/or peaking power plants increases. Energy storage can be thought of as having two components, the power delivery component, and the energy storage component. This distinction is clear for a generator and fuel tank, and it is easy to configure each component to give you what you need. In this case, the power component (the generator) is the expensive part, and it’s relatively cheap to add additional energy storage in the form of a larger fuel tank. With battery storage the power and storage components are closely linked together, meaning you can’t just add extra energy storage without adding extra power capacity at the same time.
Currently, the only economical way of achieving truly long-term electrical energy storage is using pumped-storage hydroelectricity (PSH). This stores energy by pumping water from a low lying reservoir up to another reservoir located much higher up, often in mountains. When electricity is needed, the water is allowed to flow back down to the lower reservoir, passing through a turbine that generates electricity. Round trip energy efficiency is typically 70 to 80%. There is about 127GW of PSH installed, over 99% of the worlds bulk storage capacity. Available sites to expand PSH capacity are limited by the suitable locations for reservoirs separated by significant elevation, close to population centres. As the world simultaneously develops and electrifies, annual demand for electricity could rise to 130,000TWh by 2050. Providing just one week’s worth of electricity storage would therefore require 20,000 times the current PSH storage capacity. This is not even remotely possible.
CENTRALISED HEAT PUMPS
Adoption of heat pumps in the UK is often implemented as a direct replacement for gas boilers, with individual installations in homes and commercial buildings. In such applications, it may be possible to provide some energy storage, using insulated water tanks or phase-change heat storage. However, the potential to store energy within buildings in this way is highly limited. For example, with a difference between the central heating supply and return temperature (delta-T) of 10°C, providing 10kW of heat for 12h would require over 10,000 liters of water. A 250L storage tank would provide less than 3kWh of energy storage.
Another critical advantage of large heat pumps within district heating is that it enables the use of centralised hot water storage tanks, which are much more efficient since surface area to volume ratio decreases with increasing size, and underground tanks gain huge economies of scale compared to many smaller tanks within buildings. The gigaTES project proposes deep storage pits under cities, with storage volumes of up to two million m³, delta-T of 30°C, and floating covers for as little heat loss as possible. A tank of this size could store around 75TWh of energy, enough to heat 40,000 homes for two weeks.
“There are two challenges with the construction of the pit thermal energy storages. Firstly, there is a tendency of moisture to accumulate inside the insulation. We have solved this by making a diffusion-open top cover, which allows for vapour to diffuse out of the construction,” explains Morten Vang Bobach from Aalborg CSP. “Secondly, the lid on the storage is divided into sections for improved rainfall handling. Each section has an inward fall towards the middle and towards a pumping well, which leads rainwater away that would otherwise develop underneath the lid.”
With the extreme importance of providing long-term seasonal energy storage within a renewables intensive grid, large heat pumps combined with hot water storage pits could be a critical technology.