The heat pump, in addition to being one of the most efficient technologies used as heating systems, contributes to reducing greenhouse gas emissions, and therefore is set to become one of the most common heat and cooling generation systems in the residential sector.

The integration of a thermal storage system in a heat pump improves energy efficiency and contributes to reducing the energy bill of homes and industry. In fact, CIC energiGUNE is working on the development of phase change materials (PCM) that can store large amounts of thermal energy while maintaining a constant temperature, which improves performance while reducing the size and cost of heat pumps.

Already, the Spanish National Integrated Energy and Climate Plan (PNIEC) developed by the Ministry for Ecological Transition and the Demographic Challenge foresees a 6-times increase in the installation of heat pumps in the period 2021-2030. According to data compiled by the IDAE, the heat pump fleet in 2014 reached 2 million units, doubling by 2019. With the new boost to renewables, this amount would reach 24 million units in 2030, with almost 80% of presence in the residential sector.

How do heat pumps work?

The name "heat pump" comes from their similarity to the pumps used to drive water from one level to a higher one.

Instead of water, heat pumps transport thermal energy from a cold point, which is usually the outside environment, to a higher temperature point, by a closed thermal cycle in which heat is collected by evaporating a fluid at low pressure and temperature.

The fluid, as steam, is compressed using electrical energy, increasing even more its temperature. This higher temperature heat will be used for the heating of the building. Then, as it releases its heat to the building, the fluid cools down and condenses at high pressure.

This fluid finally flows through an expansion valve where it lowers its pressure and temperature, returning to its initial state and starting a new cycle.

The operation is equivalent to a refrigerator, but in reverse, so that there are also reversible heat pumps, which can also function for cooling as air conditioners.

Advantages and challenges of heat pumps

The main advantage of a heat pump is its efficiency, which, under normal operating conditions, for every kilowatt of electrical power used in the compressor, we can get between 3 to 5 kilowatts of heat for air conditioning.

However, heat pumps that use outside air as a cold source face a major challenge from an economic and efficiency point of view. Their performance depends on the ambient conditions, especially the temperature, so the greater the difference between the outside and inside temperature, the higher is the degradation of the performance.

For that reason, at the coldest times of the day, the heat pump may not be able to supply all the power needed for heating. In such cases, combustion boilers -with higher emissions and fuel consumption-, or electric boilers -with higher electricity consumption-, are normally used as a complement to meet this occasional demand.

Energy storage systems in heat pumps

Energy storage systems are used to reduce the use of boilers and improve the performance of the installation. This allows the heat pump to operate at high efficiency during the periods when the outside temperature is higher, and stores the heat produced for using it during the cooler periods of the day.

Therefore, having a high-capacity, low-cost thermal storage system that also takes up little space is key for improving the operating efficiency of heat pumps, and thus facilitating their implementation as an air-conditioning system.

Phase change materials (PCMs) are excellent for storing energy in these applications, as they can store large amounts of thermal energy as latent heat thanks to their phase changes (solid-liquid or solid-solid) while maintaining a constant temperature during the transition process.

These systems offer much higher storage density with a narrower temperature range between heat storage and heat release than those based on sensible heat - as with water tanks, which are predominant in many of today´s building-focused applications. Because of these properties, PCMs can be used as thermoregulating materials or as heat storages for narrow temperature ranges.

The use of heat pumps as an air-conditioning system is being promoted by governments through regulations such as the Directive 2018/2001 of the European Parliament and of the Council of 11 December 2018 on the promotion of the use of energy from renewable sources. This regulation supports heat pumps for the development of renewable heating and cooling by taking into account ambient energy of aerothermal, geothermal or hydrothermal type captured by this system. Meanwhile, Royal Decree 178-2021 amending the Regulation on Thermal Installations in Buildings, establishes that the use of heat pumps to cover air conditioning, domestic hot water production or swimming pool heating demands, complying with seasonal average performance requirements, will be considered as renewable energy.

In conclusion, the high efficiency of heat pumps, especially when combined with a low-cost thermal storage system such as the one developed at CIC energiGUNE, means that they are considered the ideal heating system for both the residential and industrial sectors.