The role of heat pumps and energy storage in REACT


The transition towards a renewable energy-driven electric economy will impact all parts of our society. New technology and innovative solutions will play a major role in enabling this transition. For the electrical grid, microgeneration and the electrification of heat and transport will require adaptation of the existing supply and distribution system, from a centralised system to a more decentralised smart grid, with bi-directional flows of energy and information. A key challenge is how to deal with the intermittency of renewables, such as wind and solar power, through intelligent shifting of our energy demand patterns to match energy supply, assisted by the use of thermal and electrical energy storage. 

Heat pumps 

In a future electric economy based on renewable energy, heat pumps provide the most efficient means of using renewably-generated electricity to deliver heating and cooling in buildings. To understand how a heat pump works, it is helpful to understand the relationship between heat and temperature. The flow of heat in the natural environment is driven by temperature differences. An abundance of heat is stored in natural low-temperature energy reservoirs such as the air, the ground and water bodies such as rivers, lakes and ponds. These sources can be considered renewable, because they are heated naturally by the sun and geothermal energy.  

Heat pumps work by using electrical energy to “pump” heat from these low temperature reservoirs and deliver it as useful heating at higher temperature where it is needed. In some cases, the direction of the heat pump can be reversed to provided cooling, often consuming less electrical energy than when heating because it is working in the same direction as the natural flow of heat (in this case from a higher temperature indoor environment to a lower temperature outdoor environment). 

In buildings, most of our thermal energy demand is for space heating or cooling, to keep us comfortable, or hot water used for cooking, cleaning and washing. The geographic island locations that are the focus of the REACT project have a mix of community-owned and privately-owned buildings, each with very different requirements for thermal energy. As a result, the provision of this thermal energy requires different types of heat pumps depending on the location and building type. 

REACT demonstration sites 

The REACT project features three demonstration islands:  Inis Mór, one of a small group of islands collectively known as the Aran Islands, western Ireland; San Pietro Island, located in the Mediterranean Sea off the southwestern Coast of Sardinia, Italy; and La Graciosa Island, the smallest and least populated of the Spanish Canary Islands. These three islands each have different climate characteristics, as shown in Figure 1. 

Inis Mór is typical of a northern European climate, in that the year-round demand for thermal energy is dominated by heating in winter rather than cooling in summer. Residential buildings tend to have water-based heating systems, which deliver heat to the indoor space via emitters such as radiators or underfloor heating. Water-based heating systems are highly effective because water has a high energy density compared to air and can be used to deliver large quantities of heat in small volumes or to store for later use.  

Air-source and ground-source heat pumps are well-suited for integration with water-based heating systems in residential or commercial buildings. Air-to-water (ATW) heat pumps in particular can be easily installed as replacements for older fossil fuel boilers. For the Aran islands, ATW heat pumps have been installed in many homes in recent years as part of the Clean Energy for EU Islands initiative to reduce energy costs and the reliance of off-shore islands on fuel imports [1]. Like boilers, ATW heat pumps can also provide hot water for domestic use. ATW heat pumps are often installed with a hot water cylinder that can be heated when the cost of electricity is low, in advance of periods of peak demand. 

San Pietro Island is typical of a Mediterranean climate, with a high summer-time cooling demand. Residential homes are often fitted with window shutters to keep the inside space cool when heat from the sun is most intense, and many buildings also have electrically-powered air-conditioning systems. For small and medium-sized buildings these systems tend to be based on air-to-air (ATA) heat pumps, which cool the air directly without the need for circulating water. Heat is extracted from the air in the living space and rejected to the outdoor environment. Most ATA heat pump systems also tend to be reversible and can be set to provide heating in winter by switching the direction of the heat flow. 

In contrast to San Pietro and the Aran Islands, La Graciosa Island is located in a part of the Atlantic Ocean that experiences very mild seasonal temperatures with little variation over the course of the year; as a result, there is only a minor requirement for summer-time cooling or space-heating in winter. For this island, the majority of thermal energy demand in buildings is for domestic hot-water heating. 

Figure 1: Annual outdoor temperature variations for the three REACT demo islands, based on data from Refs [2-4]. Darker-shaded bands indicate typical daily minimum-maximum ranges, while lighter-shaded bands indicate weekly minimum-maximum ranges. 

Energy storage and demand-response 

Heat pump systems coupled with energy storage technologies allow the time at which heating or cooling energy is consumed to be offset from the time at which electrical power is generated. This is a central concept of what is termed demand-side management or demand-response, enabling the fraction of energy demand that can be met by intermittent renewable energy sources to be maximised.  

Figure 2 shows three flow-paths by which heat pumps and energy storage can be used to decouple the supply-side and demand-side of thermal energy use in buildings. Each path represents a different sequence for the generation, conversion, storage and consumption of energy.  

In Energy flow path A, electrical power generated from renewable sources is stored first in an electrical battery before being converted to thermal energy by the heat pump and delivered for end-use as space heating or cooling. In Flow path B, the energy conversion process occurs before storage, with the heat pump first converting electrical energy to thermal energy in the form of hot water, which is stored in a cylinder for later domestic use. Energy flow path C shows a sequence by which the building itself can be used as a thermal energy store, which is pre-heated or pre-cooled by the heat pump in advance of an occupied period [5].  

Each of these approaches to energy storage and management has its own technical challenges for achieving maximum effectiveness while maintaining end-user comfort and convenience. It is such challenges that provide the motivation for the REACT project. The three demonstration sites that form the core of the project will provide together a unique opportunity to implement advanced energy demand-response control strategies across a wide range of building types and geographic locations. During the validation period of the project, the REACT cloud-based platform will form the central point for gathering information to evaluate and improve the REACT ICT solution, combined with interactive feedback from the project participants. The project targets include a replication plan for scaling up the solution to meet the challenge of energy independence at the whole-island level. 

Figure 2: Energy flow paths from generation to end use using heat pumps with thermal or electrical energy storage. 

Communication and interoperability 

Central to the REACT solution will be the concept of connected homes, which we will explore further in subsequent posts. 


This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 824395.  


James Freeman, Research Engineer, Mitsubishi Electric R&D Centre Europe B.V. 

Daniel Coakley, Senior Research Engineer, Mitsubishi Electric R&D Centre Europe B.V. 


  1. European Commission (2017). Clean energy for EU islands.  
  1. National Aeronautics and Space Administration (2019). Modern-Era Retrospective Analysis for Research and Applications, Version 2. Meteorological data, Inis Mór. Retrieved from  
  1. Meteoblue. (2019).  Modelled climate data for Isla Graciosa. Retrieved from 
  1. World Weather Online. (2019). Carloforte Historical Weather. Retrieved from  
  1. T. Sweetnam, M. Fell, E. Oikonomou, and T. Oreszczyn (2019). Domestic demand-side response with heat pumps: controls and tariffs. Building Research & Information 47(4), 344-361.