HomeIndustry SectorsEnergy EfficiencyWaste heat: An untapped energy source

Waste heat: An untapped energy source

Buildings and factories are wasting huge amounts of heat into the atmosphere through air conditioning, refrigeration and production processes.

In an innovative circular economy move, Life4HeatRecovery, an EU-funded programme, aims to reverse this by using the wasted heat to supply hot water to residential and commercial customers.

This article first appeared in ESI Africa Issue 3-2019.
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The average mid-sized supermarket produces enough waste heat from its refrigeration and air conditioning units in one year to be able to provide the thermal energy needs of 200 homes over the same period. It’s not just supermarkets producing this kind of waste – there are data centres, factories, hospitals, offices and many other buildings all producing this energy as a by-product of their activities.

This wasted heat is referred to as a “pseudo renewable energy” but, despite its potential value, there are currently few ways to use it, since it is most often rejected into the environment at low temperatures – around 30 to 50°C. Apart from being a waste of a useful energy source, wasted heat also produces what is known as a “heat island effect”, which raises the temperature of urban areas to a few degrees higher than those in the countryside. Warmer cities use more energy for cooling, and so the vicious circle continues.

The concept from Life4HeatRecovery is to use the wasted energy recovered from different sources by applying it to district heating systems. The central idea is to collect heat from where it is produced (e.g. the supermarket or factory), recover it into a district heating network and then redistribute it into buildings (Figure 1).

Waste heat recovery conceptual scheme.

To do this efficiently, the district heating network distributes low temperature water (15 to 30°C), suitable to directly recover low temperature waste heat and to minimise thermal losses along the pipelines compared to conventional district heating networks (operated at 70 to 90°C). The district heating network distributes the waste heat to the door of the buildings on the network. Of course, this low temperature water is not a useful supply of domestic hot water or heating, but it can be warmed up to a useful heat at the individual buildings using heat pumps.

The concept in the European utility model

In Europe, the utility companies distribute heat through district heating networks, while electricity companies supply the electricity to run the heat pumps. Normally these two energy sources do not talk, but here is a ‘machine’ using both sources that electricity companies are getting very interested in, as this gives them a way to enlarge their markets.

For this concept, the project partners are studying several business cases to exploit the technology in a complicated, early market place. A change of paradigm is needed to move from the actual ‘monopolistic’ generation, distribution and trading structure implemented in today’s district heating and cooling networks, to a structure where multiple actors can play the role of the energy provider and where consumers can eventually profit economically from their waste heat provided to the network.

As an example, supermarkets can sell their waste heat to the district heating, but scenarios are also possible for waste heat to be collected free as this saves money in terms of less electricity being consumed to reject heat from refrigerators by means of air-coolers. Moreover, the green dividend for companies providing their waste heat for free should not be ignored, since a company’s products might become more attractive to customers looking for green solutions, if the company can demonstrate a lower environmental impact with recycling part of its energy and delivering free energy to the community.

Particularly interesting from the economic perspective is coupling the distributed heat pumps with photovoltaic electricity produced locally at customer homes: using solar electricity to run heat pumps in the day, when this is normally not used for lighting and appliances, maximises self-consumption and minimises the electricity drawn from the grid. At the moment, the project is designing the waste heat recovery plants to be used at its four demonstration sites.

Ospitaletto demonstration site

The demonstration network in Ospitaletto, in the Italian province of Brescia, is a cold district heating and cooling network that can deliver both heating and cooling services on the same pipelines, that develop over a length of 2.3km, with a total capacity installed of about 2.5MW.

The main thermal energy source is geothermal, provided by means of water wells. Customers are both public buildings (schools and gyms) and private multifamily houses. The waste heat source is a steel manufacturing company fully integrated in the urban perimeter. The total thermal energy that can be recovered from the production plant cooling towers is more than the Ospitaletto network could now deliver to its customers; therefore part of the thermal energy recovered will be used directly for the factory’s needs.

Rotterdam demonstration site

The city of Rotterdam moves from the reduction of the space heating demand of the buildings by means of their energy retrofit, which will decrease the energy use by around 50%. The remaining demand will be covered by means of electric driven devices (i.e. heat pumps) and/or by district heating. The hospital of Maasstad is already connected to the transport network to cover its space heating demand: the waste heat recovery measure foreseen involves installing a heat exchanger between chillers and wet cooling towers to recover heat at around 30°C.

Heerlen demonstration site

The city of Heerlen is located in the province of Limburg, in the Netherlands. The peculiarity of the Heerlen’s network is that it is based on a disused coal mine, now serving as a water reservoir. The thermal network includes a main ‘backbone’ connected to local clusters, through properly designed compact stations. The temperature level in the hot and cold pipes lies between 28 and 16°C. A detergent factory integrated in the urban environment including several heating and cooling plants will be tackled for waste heat recovery.

Wüstenrot demonstration site The German site is an existing innovative plus-energy district, located in Wüstenrot. This district comprises 16 newly built highly energy efficient residential detached or standalone buildings. The net zero energy supply concept combines a shallow geothermal system, heat-pumps and large photovoltaic systems. The low temperature district heating network is mainly driven by means of a large surface-near (between 5 and 10 metres) horizontal geothermal field with a size of 4,400m². Each building is equipped with a controllable heat pump and thermal storage.

South of the plus-energy district, the main sewer pipe of Wüstenrot leads wastewater to the treatment plant. The main challenge to be addressed is the development of economic viable solutions for the application of wastewater heat recovery systems in rural areas with low building density. ESI