Waste heat recovery: introductory guide

B. Individual sector overview

Waste heat recovery potential for key industrial sectors is described below. Where available, examples of operational schemes have been included.

B.1 Manufacturing

B.1.1 Cement production

The cement industry is one of the most energy intensive. Up to 45% of the total heat input to create clinker, an intermediate product to make cement, is lost ^[24].

The rotary kiln consumes the most energy, where minerals are heated to 1450°C. Exhaust gases typically pass through several chambers to recover heat before passing through the flue24.

There is also potential to capture further heat from clinker grate cooling. Cold air treats passing clinker to cool it down to an appropriate temperature, causing a heat exchange. This is significantly cooler than in the kiln at ~360°C, but still medium grade^[25].

Thyssenkrupp, a large German industrial engineering firm, use waste heat from the kiln and pre-heater to generate steam, which then drives electrical generators^[26].

B.1.2 Paper & pulp manufacturing

Most processes involved in the production of paper and pulp are carried out at ambient temperature4. However, paper drying requires a significant amount of energy and usually uses steam generation. Energy consumption patterns vary by site but the available waste heat is usually consistent.

Heat recovery is often used in-house, typically focussing on recovery from steam and flue gas economisers. However, there is the opportunity to recover further medium-grade heat (>150°C) from these mills, by employing further heat recovery methods4. Scottish paper and pulp sites are usually located near areas of medium heat demand (>250 MWh), although not necessarily close to existing district heating networks4.

UPM-Kymmene’s facilities at Kaukas, Finland, comprises of two paper mills and a pulp mill. The Kaukas operations produce 590,000 tons of paper annually. Heat is recovered from cooling processes using a system of over 100 heat exchangers and recycled through the plant. Between 10-20% of energy recovered at the mill is used elsewhere on the site and the surplus is sold as process steam to the local paper and sawmills^[27].

B.1.3 Food & drink manufacturing

Production processes and waste heat output vary significantly across the food and drinks sector. Dairy, baking, and brewing sectors are the predominant industries in Scotland4.

Heat sources are mostly condensate arising from evaporation, distillation, and cooking processes, alongside boiler exhaust gases of a medium grade (typically between 120 -150°C)4. Food and drinks plants are often close to urban areas, offering the opportunity to reuse heat in the surrounding area, either for industrial or domestic use4.

Tennent’s brewery in Glasgow recovers waste heat from their Anaerobic Digestion (AD) plant and chimney stack. Heat exchangers upgrade heat from brewing process effluent to heat incoming material to the AD plant, reducing its heating demand by around 400 tCO₂/annum. A boiler economiser was also fitted to the flue, redirecting gases to pre-heat the water entering the boiler. This reduces the required increase in temperature and cuts carbon emissions by 500 tCO₂/annum^[28].

B.1.4 Distilleries

Waste heat in distilleries is typically between 70-90°C. Seasonal and temperature variation is generally low, offering a consistent source of heat throughout the year4.

The most considerable potential for heat recovery is from heat lost to the surroundings via a flue4. Utilising this heat in other steps of the distilling process is usually done using a heat pump. This can save up to two thirds of the heat energy used in the still house^[29].

Micro-distilleries account for 90% of UK distilleries^[30]. For micro-distilleries, specialist heat recovery systems are often too expensive and not designed for their needs^[31]. As such, significant research and development is taking place to identify opportunities for small scale heat recovery. These methods are generally centred on the use of heat pumps and heat exchangers to capture waste heat from the various stages in the distilling process^[32],[33].

At Bowmore Distillery, on the Isle of Islay, waste heat from the drying process and stills is upgraded using a heat pump and reused in the visitor centre and the neighbouring community swimming pool^[34].

B.1.5 Chemical manufacturing

Globally, the chemical industry is the largest industrial energy consumer. It is a diverse sector which requires heating for activities such as: distillation, curing, boiling, drying, cooling, transportation, and storage.

Typically, waste heat from chemical manufacturing is of a lower grade and arises from cooling water or exhaust gas from boilers. This is often recovered and used onsite.

Solutia UK, based in Wales installed a condensing economiser waste heat recovery unit on their Combined Heat & Power (CHP) facility. The predicted project savings were 1.1 MWh and 1400 tCO₂, with a payback period of 2 years^[35].

B.2 Services

B.2.1 Energy from Waste

Energy from waste is the generation of energy in the form of electricity and/or heat from waste treatment. Scotland has the capacity to process over 1.4 million tonnes of waste per year at Energy from Waste (EfW) sites with another 1.1 million tonnes of capacity in planning or construction.

The majority of Scottish EfW plants are either conventional incineration or Combined Heat and Power (CHP). In these plants non-recyclable treated waste is burned in controlled conditions at high temperatures (often above 850°C), creating a high-grade source of heat ^[36].

There is also the opportunity to recover lower grade heat from incinerators, such as that which is residual in the outcoming bottom ash. However, this is generally overshadowed by the quantity of available high-grade heat.

The Scottish Government commissioned an independent review of the role of incineration in the waste hierarchy in Scotland. The review recommended deploying combined heat and power for as many existing incineration facilities as possible.

SEPA, Scotland’s environmental regulator, requires all EfW permit applications to demonstrate at least 20% energy recovery. This must be accompanied by a Heat and Power Plan, to show how, within a period of seven years from commissioning, and where practicable, they will connect the facility to a heat network^[37].

In Shawfair, Midlothian a joint venture by Midlothian Council and Vattenfall Heat UK known as Midlothian Energy Limited (MEL) will supply over 3,000 homes, as well as education and retail properties. This project is under construction and will be powered by waste heat from the Millerhill Recycling and Energy Recovery Centre. This initial phase is expected to save more than 2,500 tonnes of CO₂ a year^[38].

B.2.2 Industrial laundries

The operations carried out at industrial laundry sites are energy intensive and there are several processes which produce waste heat.

For example, the washing and cleaning of textile products produces waste heat between 80-120°C with low seasonal and temperature variation4.

Up to 40% of the energy used is to heat water for washing. Heat from wastewater produced as part of the washing process can be recovered and used to preheat incoming cold water, reducing energy consumption. This principle can also be applied to the warmed air used in the clothes driers^[39].

Heat pump dryers are more energy efficient than traditional tumble dryers. Heat pump dryers use refrigeration cycles to capture and reuse heat from the drying process, instead of venting to the surroundings.

Heat can also be recovered from exhaust air from any part of the operations, including steam heat boilers and combined heat and power systems. This heat can either be used within the laundry operations or utilised as space heating for the site^[40].

At the Victoria Hospital in Kirkcaldy, the laundry was losing the equivalent of £57,000 of heat a week from a wastewater tank before implementing heat recovery. Outgoing water is now used to preheat water coming into the washer. This reduces the need for steam in the washing process and therefore the level of moisture retained. As a result, drying times have reduced by 25%, increasing output, and saving energy. The payback period of this project was just over a year^[41].

B.2.3 Electrical substations

Transformers require a cooling circuit, usually utilising oil, to function safely and efficiently. This is achieved by placing the transformer core in an oil bath to remove resistive heat generated by the coil windings, with typical waste heat around 45°C. A conservative estimate of waste heat potential across a year is more than 31 MWh for a typical substation transformer4.

The higher the electrical load, the more heat energy the transformer will produce. The recovery of heat from the oil is relatively simple and just requires an additional heat exchanger and pump.

Substations are often spatially constrained sites and access is strictly controlled for safety reasons. Therefore, detailed planning is essential to ensure feasibility. As with other waste heat sources, it relies on suitable and reliable heat consumers to be nearby to be financially viable.

SSE has been collaborating on a trial scheme with the National Grid to use the waste heat from transformers to generate hot water and heating. Testing of the scheme began in 2021 in Deeside, Wales. If successful, this could be rolled out across the National Grid’s 1300 substations^[42].

B.2.4 Wastewater treatment plants

Wastewater Treatment Plants (WWTP) treat wastewater which is continuously available and in large quantities, allowing for low-grade heat recovery.

Whilst the flow rate tends to remain stable through the year, effluent water temperature can vary significantly. External conditions influence the temperature and consequently, the amount of waste heat captured in winter will be lower than during summer4.

Many of these plants are near significant heat demand and/or heat networks. In urban areas, such as Edinburgh and Glasgow, the substantial heat demand, high capacity, and flow rates at WWTPs increase this sector’s potential4. Modelled heat supply data for WWTP is provided by Scottish Water as part of the Scotland Heat Map dataset (please refer to section 5.5).

However, there are a range of technical challenges that arise from heat recovery from effluent water as flows can fluctuate greatly and a minimum temperature must be maintained within the effluent water to enable effective treatment4.

In Renfrewshire, a fifth-generation, heat network converts treated water into low temperature heat. Treated water, otherwise flowing into the White Cart River, is directed into the Scottish Water Laighpark Energy Centre. Here heat from is extracted from the water before going through 3.7 km of underground pipes to the Advanced Manufacturing Innovation District Scotland (AMIDS). Heat pumps at each building upgrade this heat to suitable temperatures for use. This network supplies 90% greener heat and hot water than a traditional gas boiler facility^[43].

B.2.5 Sewage pipe network

In this case, heat is taken from the sewage pipes upstream of any WWTP works. Scotland’s sewers transport over 920 million litres of sewage every day and are estimated to contain enough heat to warm Glasgow for more than 4 months a year^[44]. Sewage water temperatures tend to range through the seasons, between 10°C in winter and reaching peaks of 25°C in summer, averaging out across the year at 15°C^[45].

Scottish Water publishes data on the flows within their waste water network which can be used to assess waste heat potential and is available here: Waste Water heat extraction opportunities (arcgis.com)

Borders College in Galashiels has installed a sewage heat recovery system. This project comprises of a heat pump connected to a district heating network. This provides 95% of the heat needed by the campus, equating to around 1 GW per annum^[46].

B.3 Other

B.3.1 Hydrogen electrolysers

Electrolysers can be used to produce hydrogen. This method uses an electrical current to separate water into hydrogen and oxygen. The electricity used must come from renewable sources for the hydrogen to be considered ‘green’.

Electrolyser efficiency can be up to 80%, however substantial waste heat is still generated. Electrolyser cooling is essential to improve efficiency and cell lifetime. Therefore, coolers are usually placed on the top of electrolyser containers, dissipating heat of 50-80°C into the surroundings^[47].

As decarbonisation is an essential component of the development of electrolyser technology, waste heat reuse is being considered as an integrated part of plant design. In Hamburg a scheme expected to be operational by 2025 will harness waste heat from a 100 MW electrolysis plant for a DHN^[48].

Waste heat reuse opportunities would be dependent on the electrolyser location. This could suit industrial offtakers located nearby, or the heat could be distributed further afield through a DHN.

B.3.2 Hydrogen fuel cells

Fuel cells can be applied to provide power across various sectors, including transport, industrial/commercial/residential buildings, and energy storage for the grid. A typical fuel cell has an efficiency of around 60%, where much of the lost energy is dissipated as heat ^[49].

Proton Exchange Membrane Fuel Cells (PEMFC) are the most common technology used in vehicles and vessels. PEMFCs typically reject heat at 75-80°C49. Other technologies have high grade exhaust heat, such as Solid Oxide Fuel Cells (SOFC), where temperatures are typically between 600-1000°C^[50].

The Event Complex Aberdeen (TECA) hosts the largest fuel cell installation in the UK at 1.4 MW electrical generation capacity^[51].

B.3.3 Supermarkets

Supermarkets offer a continuous flow of low-grade heat, typically between 20-40°C, which is rejected from the cooling / refrigeration systems in display cabinets. Furthermore, larger supermarkets may have a centralised refrigeration system which could offer low-grade heat between 60-90°C4.

Heat pumps could be used to boost these temperatures to enable heat to be supplied to district heat networks. However, the development of newer DHNs operating at ambient temperatures, known as ‘5^th Generation Networks’, would allow for the direct re-use of low-grade heat.

Most supermarkets are located close to dense urban areas allowing waste heat recovered to be easily delivered to homes through a DHN.

Bals Elektrotechnik (BALs), an electronics supplier in Denmark, have equipped their store with CO₂ based refrigeration units and incorporated heat recovery. This technology recycles 95% of heat recovered and has saved the store 70% on its annual heating costs and 37% on its electricity. The recovered heat is used to heat the store, its hot water and supplies 15 homes via a DHN^[52].

B.3.4 Data centres

The data centre industry is growing rapidly, and due to its energy intensive nature, so are calls for waste heat recovery to be an integral part of their design.

To prevent electronic components becoming damaged due to overheating, cooling systems are used to cool the circuits to around 25°C, with the waste heat exhausted into the atmosphere. For air cooled systems this heat can be recaptured at 25-35°C, whereas liquid submersion-cooled systems this is around 50-60°C^[53].

In Stockholm, Sweden, an initiative was launched to attract data centres to the city and capture their excess heat for the city’s DHN. Since 2022 more than 100 GWh of heat has been recovered. Heat pumps are used to boost the waste heat from cooling the processors to temperatures which can be delivered to the DHN. This system heats the equivalent of 30,000 modern apartments^[54].

B.3.5 Edge computing

Edge computing uses decentralised data centres, some of which may be in households, rather than servers in a large central data centre.

As the need for increased processing speed grows, edge computing could offer a more self-sufficient way for buildings to incorporate communal heating – integrating the common need for heat and data. Through providing heat in-house, buildings would not need to be located on a DHN, presenting more sustainable growth opportunities outside a city centre.

In Devon a washing-machine-sized data centre provides 60% of the hot water needs of a public swimming pool. The oil used to cool the processing units is pumped through a heat exchanger, which in turn warms the pool water to 30°C^[55].

Frequently asked questions

1) What is “waste heat”?

Waste heat is heat produced as a by-product of another process. This heat is not directly used in the process itself (hence “wasted”) and can often be captured and used to reduce overall energy consumption on-site or exported to a potential buyer.

2) Why is it important to investigate recovering waste heat from our industrial and manufacturing processes?

Waste heat recovery benefits are twofold. Firstly, depending on the grade and availability of waste heat and available offtakers, there is an economic incentive to save on fuel costs or generate an additional revenue stream. Secondly, depending on the source of waste heat, it could contribute to achieving UK’s decarbonisation goals by displacing carbon-intensive fuels.

3) What are our options to turn our waste heat into something useful?

This largely depends on the specifics of the process that results in waste heat, and the temperature of the heat. Options for reuse include either using a waste heat capture technology (e.g. a heat exchanger); converting it to electricity; reusing it on site; or exporting to a district heat scheme or nearby demand.

4) Can we use the recovered heat within our facility or is it better suited for other purposes?

There are a few options to reuse waste heat. The first, and most common, is capturing and utilising it in industrial processes to reduce fuel costs. An example of this is using an economiser to raise the temperature of boiler feedwater.

The second is using this heat to provide heating and hot water or electricity to a site. Note. converting to electricity requires waste heat to be at higher temperatures than for space heating.

The third is selling captured heat to a potential buyer, typically a DHN operator. It is also possible to export electricity to the National Grid or to another consumer if electricity generation is an option.

All options depend on the amount and grade of recoverable heat, the economics of fuel prices and if there are offtakers available nearby. Comparing these options often requires further investigation using lifecycle analysis.

5) Are there any rules or environmental concerns we need to consider when recovering waste heat?

Mainly, any concerns would be to ensure no damage comes to the immediate environment in the process of recovering this waste heat. For example, extracting heat from a river could reduce the water temperature to such a degree the local ecosystem suffers.

6) What kind of equipment or changes would we need to make to start recovering waste heat?

Typically, a pipe system is needed to divert the wasted heat to a new heat capture unit. In some processes this may already be available (e.g. flue stack). After the waste heat capture unit, an additional distribution network may be necessary to transport it to its desired location.

7) Will incorporating waste heat recovery disrupt our current operations?

This largely depends on the operation in question, but it would be likely some partial or complete shutdown would be required to install the required equipment and distribution network.

8) How long will it take for us to see a return on investment from recovering waste heat?

This is largely situation dependent and factors such as: the volume and grade of recoverable heat, the -economics of fuel prices and if there are offtakers available nearby.

9) Is there any funding and general advice available?

Yes, there are multiple funding schemes which may be applicable to a project. These include various funds to support heat networks, as well as advice from departments within the Scottish Government. Please see Section 4.7.

10) Are there any downsides or risks we should be aware of?

The two categories of risk are technical and commercial. Technically, it is important to ensure the waste heat recovery system does not impact usual operations.

Commercially, it is important to ensure the waste heat system will make a return and/or achieve carbon cutting objectives. This should be assessed via a lifecycle cost analysis. This activity is usually conducted in the feasibility stages (i.e. RIBA Stage 2 and/or CP1 Stage 2).