Nov-2022

The heat-pump way to more sustainability

The heat pump as an industrial technology has been mature for decades, but only now with sustainability demands and price of fossil fuels is it coming of age.

Rasmus Rubycz
Atlas Copco Gas and Process

Article Summary

The industrialisation of much of the world over the past few centuries has been predicated on an abundance of fossil fuels as energy sources. And until the last 40 or 50 years, whether coal, oil, or natural gas, this was hardly seriously questioned. It is easy to figure out why: cheap energy powered the modern world and helped develop modern consumer societies, first in Europe and the US, then further afield. There was, however, a major price to pay: burning fossil fuels to power modern industry is the main cause of climate change, experienced in extreme weather, poor air quality, and species extinction across the planet.

Nevertheless, if albeit belatedly, attitudes to fossil fuel use are changing, and not just because climate demonstrations have put greater pressure on governments, businesses, and industries. For the first time in modern history, the goals of many politicians, business leaders, and climate activists are aligning: the consensus is that green sources of energy must replace fossil fuels if we are to continue to power modern societies.

At the same time, the reasons and motivations underpinning this growing consensus vary, with some people more concerned about protecting the planet, for example, and others more focused on the continued existence of a specific business or business model. But whatever the reason, they all agree that the result must be a rapid transition to a more sustainable way of living.

End dependence on fossil fuels
While notions of sustainability have long underscored the reasons for providing green energy sources, more recently it has also become about efforts to minimise the effect of geopolitical crises, volatile prices, and disruptions to fuel imports. Green hydrogen, for example, is one much-touted alternative energy source, something frequently depicted as a transformative solution to help end dependence on fossil fuels. Often overlooked, however, is that it is produced from electricity with high energy losses, and it still requires greater efficiencies before its wide-scale deployment. Similarly overlooked is that in many situations heat pumps can do the job as well as hydrogen does, and even increases in electrolyser efficiency will not change this fact.

That is not to downplay the potential of hydrogen, and there are many industrial applications for which it can be used sensibly, such as with high temperatures for combustion, direct chemical reaction, or long-term energy storage. Indeed, the decentralised production of hydrogen on-site in chemical and petrochemical production plants offers upcoming opportunities to not only use the produced green hydrogen, but also to use the unavoidable waste heat.

Temperature disparity
While there are industries and processes that require high temperatures, there is also a lot of demand for energy below such high levels. Unfortunately, we usually use extremely hot flames to generate even low temperatures, even though it is not necessary. Temperatures of only 100-250°C are required in paper production, district heating, the food industry, and parts of the chemical industry, for example (see Figure 1).

Because in many situations there is a disparity between the temperatures used and those actually required, the potential of heat pumps is gaining greater attention. A heat pump functions in a similar way to a normal refrigerator (see Figure 2): a liquefiable gas (the refrigerant) is evaporated in a cyclic process at low pressure, compressed in a compressor, and condensed at a higher pressure. A pressure reduction component, such as an expansion valve, closes the cycle, while during evaporation the refrigerant absorbs heat, typically from inside the refrigerator or from a low-temperature environmental or process heat source. The gaseous refrigerant condenses after compression at high pressure and high temperature. In the case of the refrigerator, this heat is released into the ambient air. With a heat pump, this heat is put to practical use, ranging from process heat to district and domestic heating.

The COP and getting the most from the electricity
One significant advantage of the heat pump is that it generates much more usable heat from the same amount of electricity compared to other technologies. In fact, no CO2 is emitted if green electricity from the sun, wind, water, nuclear power, or other sources is used.

It works by the heat being loaded into a refrigerant and raised to a higher temperature level using additional energy. The ratio of electricity used to usable heat is referred to as COP (coefficient of performance), and for a heat pump it typically means that 2-4 kWh of heat can be pumped with 1 kWh of electricity. Electric heaters have a COP of 1, and the electrolysis and combustion of hydrogen are typically 0.6, mainly due to losses in electrolysis (see Table 1).

The long farewell to fossil fuels
The first large-scale heat-pump installations were operated in Switzerland, back in 1938, and they provided a solution to minimising dependence on imported coal, which resonates with the current challenge.

Heat pumps offer enormous potential to save CO2, as seen in Scandinavia today, where several large Atlas Copco Gas and Process heat pumps using turbocompressors have operated since the 1980s. The setup works with wastewater from a sewage treatment plant which acts as the heat source, while the heat sink is the urban district heating system. The systems have thermal outputs of over 60 MW per unit.

Furthermore, two Atlas Copco Gas and Process heat pumps installed in Stockholm's heating network, each with 40 MW thermal output, save 90,000 metric tons of CO2 emissions annually (compared to the previous use of heating oil). To achieve a comparable saving in road traffic, it would mean that the average Swedish gasoline-powered car would have to drive 500 million fewer kilometres every year.

Conversion to energy circular economy
From the many application segments in which heat pumps are used, five stand out: the production of paper, food, chemicals/petrochemicals, the aforementioned district heating, and more general manufacturing where heat is required. The required heat for these is typically between 80 and 250°C. At the same time, low-temperature waste heat is available in all production plants, which is rejected via cooling towers. From an energy point of view, this conventional use of heat is an open process that can be converted into an energy circular economy using a heat pump.

But it is not just in the realm of industrial production where heat pumps are employed. In fact, there are more heat sources that are readily available, which may not be immediately recognisable as such: municipal sewage treatment plants, and hydrogen electrolysis plants, for example, represent a continuous flow of low-temperature heat that can be made usable again. Moreover, new megawatt-scale electrolysers are being installed almost weekly in many places around the globe, representing a huge, untapped potential of low-grade waste heat.