Electricity network constraints and the 2024 New Build Heat Standard: research

A3 Zero emission heating options

A3.1 Resistance heating

Resistance heaters create heat by running an electric current through a resistor. The resistor has a low efficiency (compared to other forms of electric heating) and proportionally converts one kWh of electricity to one kWh of heat. This means that fulfilling a significant heating demand requires an equally significant power load.

Resistance heating has low capital costs due to their cheap parts and simple installation. The caveat is that the rate of electricity conversion causes a high operating cost. It is for this reason that resistance heaters are best suited for properties that have low space heating demand by either having a small floor area and/or having a high-performance thermal envelope. In such circumstances, the domestic hot water becomes the driving heat requirement.

There are various ways the technology is applied in order to provide space heating or hot water.

A3.1.1 Convection heaters

Much like traditional home radiators, convection heaters use heating elements to heat air within a property. Heat is generated the moment the heating elements have reached temperature. Hot air is less dense than cool air and so rises due to its buoyancy. The hot air is continuously replaced with colder air yet to be heated. They can come equipped with fans to speed up airflow. Whilst they can be wall mounted, they are often sold as free-standing units to supplement inadequate home heating.

The timing of electricity load is determined by the heat demand. Use of multiple heaters simultaneously can trip a domestic property ring circuit by exceeding the maximum designed electrical loading of 23kW.

A3.1.2 Immersion heaters

Immersion heaters use their heating elements to heat water in a hot water cylinder. They often come as standard with hot water cylinders to be used as a primary source of water heating or to provide a back-up to primary heat sources such as conventional boilers and heat pumps.

In the case of air source heat pumps, immersion heaters are a necessary companion technology to provide pasteurisation cycles at temperatures unreachable by heat pumps.

Water tanks are well insulated and act as a thermal store, retaining heat until its future use. A benefit of this is that demand can be managed so that heating is completed overnight when grid demand is low and consequently multi-tariff electricity prices are too. Immersion heaters typically demand a minimum of 3kW by design and therefore have a significant impact on a property's load at the time of use.

A3.1.3 In-line heaters

In-line heaters can provide space heating and/or hot water by heating water. Rather than heating water in a cylinder, in-line heaters will heat water in transit. They are integrated into the water flow pipeline and use an electric element to heat water en-route to its point of use.

They can be used with or without hot water tanks but are more often used when there are space constraints, meaning there is no tank.

A3.1.4 Night storage heaters

Night storage heaters are convection heaters that have an in-built thermal store. They contain thermal bricks that are made of high-density materials. The thermal bricks are heated over night when grid demand is low and consequently multi-tariff electricity prices are too. The thermal bricks radiate their internally stored energy throughout the day, preventing or reducing the need for the unit to provide convection heating during the day when electricity prices are higher.

A3.2 Heat pumps

Heat pumps operate by taking advantage of the vapour compression refrigeration cycle. This technology has been used commercially in refrigerators and air-conditioning units since the 1850s and has been repurposed in more modern times to provide heating.

A heat pump passes low pressure, low-temperature refrigerant fluid through a heat exchanger that draws heat from an external heat source that evaporates the refrigerant. The refrigerant gas is then passed through an electrically driven compressor to compress it, increasing its temperature and pressure. The heated gas is then passed through another heat exchanger where heat is transferred to a transfer medium to be emitted within the property or hot water tank. The transfer of heat also causes the refrigerant gas to cool and condense back into a liquid. The warm liquid is then passed through an expander to lower the pressure which causes a drop in liquid temperature. The low pressure, low-temperature liquid is then ready to begin the cycle once again [9].

The unit's compressor is the key source of electricity demand. The higher the temperature difference between the heat source and the target heat, the harder the compressor will work and the more electricity it will require. Therefore, the temperature of the heat source, and the temperature required at the end use have an impact on system efficiency. The end use could be for hot water which commonly requires the maximum of the heat pump to reach temperatures of 55 °C or for space heating which is dependent on the heat requirement of the emitters to meet the properties target room temperature. Heat emitters are discussed further in A3.2.2.

A3.2.1 Heat sources

There are a variety of sources that heat pumps can be configured to extract heat from. The different sources range in temperature and therefore are a key consideration for system operation and efficiency. They are described below:

• Air source – These heat pumps extract heat from the outdoor air. They are relatively cheap and straightforward to install when compared to other heat sources described below and are also the most commercially mature. A disadvantage is that air temperature can vary significantly month to month and so the system will have to work harder in the winter months to raise the temperature of the refrigerant to what is required, therefore, the unit is often least efficient when it is needed most. In circumstances where outdoor temperatures risk freezing the refrigerant, the heat pump may enact defrost cycles to prevent it, also reducing system efficiency.
• Ground source – These are configured to collect heat from the ground through heat exchangers in horizontal trenches or vertical boreholes. Boreholes take advantage of ground heat that increases in temperature with depth. For horizontal collectors, the area that heat is extracted from is replenished by solar gain or rainfall. For both systems, the general consistency of temperature provides a reasonably constant efficiency of the heat pump cycle no matter the weather conditions.
• Water source – Water source heat pumps perform similarly to ground source heat pumps due to the consistency of the temperature. The circumstances where heat pumps have access to water often mean that water cooled by the compression cycle is quickly replaced by warm water through convection or from natural flows such as those of a river.

A3.2.2 Transfer mediums

Heat pumps can provide heat to properties through two different transfer mediums. Heating through air is most common in commercial buildings where cooling may also be required. Ducting systems with fans circulate the warmed air around the building and distribute it where it is needed.

Water based systems are more common in domestic properties that typically only require heating. The heated water is transferred through piping around the property and delivered to radiators or underfloor heating systems to emit the heat. The more surface area the emitter has, the lower the temperature that is required to emit an equivalent amount of energy. A lower target temperature for a heat pump requires less work from the compressor, therefore increasing system efficiency. As underfloor heating systems have a large surface area, they are typically the most efficient option for space heating. There is a significant opportunity for low cost integration during the building of new developments.

A3.2.3 Hybrid heat pumps

Hybrid heat pumps combine heat pumps with conventional boilers. The heat pump component may be sized so that it only meets the requirements for the majority of the property's heating demand and leaves the conventional boiler to facilitate the remainder of demand or the hot water tank specifically. Most hybrid heat pumps combust fossil fuels in the conventional boiler but as the energy transition gains momentum their fuels may be replaced with low carbon alternatives such as biofuels or hydrogen.

A3.3 District heating

District heating systems take advantage of economies of scale to efficiently deliver heat to properties and decrease the dependency of properties on individual technologies. Heat networks can range in size from a shared system amongst a few properties to multi-technology systems interconnected between entire cities.

A heat network delivers heat through water transmitted through insulated pipes. Individual dwellings will tap into the heat network and extract the heat that they require without having to generate any themselves. In cities where dwellings are likely to have space constraints this is particularly useful as floorspace or storage space is relieved that would otherwise house a hot water cylinder or gas boiler. The amount of heat consumed within a property is monitored and invoiced much like any other utility bill.

The source of heat ranges depending on the system but can include waste heat from industrial processes, solar heating, heat pumps and conventional combustion boilers.

A3.3.1 Thermal stores

Thermal stores can be integrated with any technology but are more commonly associated with district heating systems where heat generation is not driven by heat demand. Thermal stores are well insulated storage units that will offset the heat supply until it is demanded. In the case of electrified heating, thermal stores can also be advantageous for generating heat when demand on the grid is low and subsequently electricity prices are cheap, thereby reducing the operating cost of the system.

A3.4 Emerging technologies

There are novel heating technologies that are not included in the project scope due to their commercial immaturity. This section provides a brief overview of those technologies that are worth consideration in the medium to long term future.

A3.4.1 Phase change materials for thermal storage

Phase change materials are often denoted as "heat batteries". The heat loss is low over time and release can be triggered whenever the owner requires it. They are particularly well suited for small apartments due to their high volumetric latent heat storage capacity that allows them to take up a small space footprint.

A3.4.1.1 Sunamp

Sunamp is a company that is providing a phase change material solution in the present future. They use primarily solar panel generated electricity to transfer heat into a phase change material that integrates with existing heating systems. Their size is tailored to the needs of the system and can be expanded if needed [29].

A3.4.2 Heat recovery ventilation

Highly efficient building envelopes, such as those that conform to "Passivhaus" standards may use heat recovery in their ventilation systems. The system passes extracted air through a heat exchanger to preheat air being drawn into the building. This decreases heat loss during air ventilation to further increase energy efficiency of a property. Heat recovery ventilation units may become a common solution among property developments if building energy efficiency regulations reach a high enough standard.

A3.4.3 Infrared heating

An infrared heater transfers energy to an object with a lower temperature through electromagnetic radiation. Depending on the temperature of the emitting body, the wavelength of the peak of the infrared radiation ranges from 780 nm to 1 mm. The method of transmission of energy allows for a wider spread of heat than can be achieved with convection heating and provides longer term warming of objects within the room.

A3.4.4 Transcritical heat pumps

Transcritical heat pumps use carbon dioxide as the refrigerant gas. They have the same basic configuration as any heat pump. However, due to the different properties of CO2, a much larger temperature difference across the heat distribution system is required. This means that transcritical heat pumps require a low return temperature (usually below 30 °C) but can supply high flow temperatures (over 80 °C). They may be suitable for heating hot water storage tanks or high temperature requirements in industry.