Surplus Heat Utillization
The ability to exploit waste heat as an energy source for other processes requires energy storage technologies for both short- and long-term applications. We are working to achieve the optimal integration of such technologies and systems within the overall energy system. This applies in particular to processes in which the supply of waste heat is either unpredictable or transient, such as the casting and drawing-off processes carried out in the metals industry.
All industrial processes will be encumbered by energy losses. This means that we have to supply more energy than we otherwise would in an ideal system where energy losses are zero. For the most part, the losses are in the form of waste heat, such as that contained in the exhaust gases generated by the process in question. As is shown in Figure 3, there are in principle three possible ways of exploiting waste heat. Direct use is often the least expensive option once an internal or external source of demand is recognised that can exploit the heat at the temperatures at which it is immediately available. An example is supply to district heating systems. However, it is often the case that large volumes of waste heat cannot be utilised directly.
One possibility here is to raise the heat to a higher temperature that coincides with the demands of another process, or an external consumer. For example, heat at 80⁰C may be exploited as a source in a heat pump that in turn supplies heat to produce steam at 120⁰C – steam that is probably produced today either by the combustion of fossil fuels or directly by electricity. However, there is a great need for the further development of industrial heat pumps that can operate at such temperatures. Key focus here is on the development of enabling technology concepts, costs, efficiency, reliability, and the use of work media that do not, if emitted or discharged, result in negative impacts on the external environment. In the case of the latter, the HighEFF centre is focusing on the utilisation of media categorised as natural and eco-friendly, which occur naturally in the biosphere cycle, such as water, CO2, ammonia and hydrocarbons. In this way we can be sure that discharges or emissions to the external environment will not result in long-term, unwanted and harmful environmental impacts such as CFCs and HFCs have been shown to do.
However, it is frequently the case that there is no need to supply heat either directly or in a temperature-upgraded form. In such cases, the best option is to exploit the heat as a heat source in a heat-to-power process. The most obvious of these are Rankine cycle systems, such as are used in the recovery of heat to drive the high-pressure evaporation of a work medium, e.g. for turbines. In this case, electricity is produced by a generator that is connected to the turbine used in the process in question. Electricity provides full flexibility in terms of utilisation within the same industry. Alternatively, it can be fed directly into the distribution grid. However, such a system is constrained by the temperature at which the heat is made available, by the investment needed to develop the plant, and by the prices of electricity from alternative sources. Currently, a plant with megawatt class capacity that exploits heat sources at temperatures above 400⁰C is able to be economically viable. However, there are very large volumes of waste heat generated by Norwegian industry in the temperature range 80 to 250⁰C7. In many cases, the heat is contained in contaminated gas from which it is difficult and potentially expensive to extract.
The development needs being addressed by the HighEFF centre are typically in fields related to:
- the development of reliable and inexpensive heat exchangers used for heat capture
- more efficient and inexpensive plant that utilise natural work media
- alternative and efficient process technologies
Energy storage in an integrated energy system
The introduction of intermittent renewable energy sources has greatly boosted demand for energy storage technologies. Key concepts here are flexible hydropower sourcing, batteries and hydrogen. In industrial processes, thermal energy storage is often highly relevant, and this constitutes the main focus of activities carried out at the HighEFF centre. Thermal storage can enable the utilisation of energy in heat over a wide range of temperatures – from as high as perhaps several hundred degrees C, to as low as -50⁰C in refrigeration units.
Refrigerated storage is highly relevant in the foodstuffs sector. The levels of demand for cold storage will often vary according to the time of day. Currently, refrigeration units are often designed for peak demand. By charging a refrigeration unit when demand is low, and utilising it to meet peak demand, enables peak design output to be reduced during periods of low demand. This results in lower investment costs and increased plant efficiency because it can be operated at a stable high capacity. In the near future, it may also be relevant to introduce time-differentiated electricity prices that vary during the course of a 24-hour day. The deliberate exploitation of periods when prices are low to charge the refrigeration unit will result in reduced energy costs. A key success criterion here is the development of refrigeration technologies that are both efficient and sufficiently inexpensive.
Thermal storage units that operate at higher temperatures can be exploited similarly in processes, e.g. the capture of heat from intermittent sources such as that generated from foundry products or solar heat sources. Heat from the storage unit can be used to balance supply in response to demand, as in the case of a refrigeration unit, or to meet more constant levels of demand, for example in connection with steam production for an industrial process or electricity generation in a heat-to-power process.
One energy storage technology currently under development is the so-called PCM (Phase Change Materials) approach. Available energy is stored in a material that has been converted to a higher energy phase. The energy can then be released when the material is returned to its original state. The simplest example of such a material is water, which exhibits freezing and boiling points within the ranges that are practically applicable in some industrial processes. However, more advanced materials are being developed that are adapted to specific applications and temperature ranges8.