While physics tells us that energy cannot be destroyed, it also tells us that energy naturally becomes less useful. Electrical energy and mechanical work are dissipated through resistance and friction, becoming heat. Remember that heat is a quantity of energy, not a measure of temperature. High temperature heat spreads out to become low temperature heat, with the same total energy. This process is sometimes referred to as entropy (which is not quite the same as the entropy that mass has in thermodynamics). Heat becomes more difficult to use as it spreads out and its temperature decreases, despite it still having the same energy. This means that it is often not practical to recover energy and the efficiency of many common processes remains low.
“20-50% of the energy used in an industrial facility is waste,” says Damir Naden, energy consultation and sales manager, Enbridge Gas Distribution, Ontario, Canada. He continues: “Energy is being converted, distributed and used, and at each step there are losses. Maybe you take natural gas and create steam, or you take electricity and create compressed air: anyway, energy is converted, then energy is being distributed around your place and energy is being used.
“When you talk to people about heat recovery, the first thing that they think is they take the heat from the boiler’s flue gas and they recover it… it’s a good first step; the heat is medium grade; it’s always there, as long as the boiler is on, and it’s very easy to determine how much energy you actually waste in your steam boiler.”
Below are considered three common ways we might want to use heat. First is space heating. We typically heat buildings by heating the surface of radiators. The heat transferred to the room is proportional to the surface area of the radiator multiplied by the temperature difference between the radiator and the room. So the lower the temperature of the radiator, the bigger it needs to be. If we have a supply of heat at a temperature just a few degrees above room temperature, the radiators would need to be impractically large to provide any useful heat.
Central heating boilers initially heat water from room temperature to the inlet temperature of the radiators. However, once the water is circulating the return temperature is still quite hot, with a delta T of several degrees. This means that low temperature heat generally cannot be used to preheat the return into the boiler either.
The second use of heat is in engines. Heat can be used to produce mechanical work, for example, in internal combustion engines in cars or steam turbines in power stations. These generally involve heated gas expanding and pushing on mechanical elements. Low temperature heat is always rejected, either while condensing the fluid in a closed cycle or through other means.
The efficiency of all heat engines is limited by temperature drop across the cycle, with engines operating at higher temperatures therefore achieving higher efficiency. In a theoretically perfect heat engine (Carnot cycle), with no mechanical losses, the energy efficiency is a function of the maximum hot temperature (T2) and the minimum cold temperature (T1) in the cycle. If the temperatures are given in degrees Kelvin, the efficiency equals (T2-T1)/T2. So, if the lower temperature is 20°C and the upper temperature is 600°C the maximum theoretical efficiency is 66%, but a real engine will have a much lower efficiency. If the upper temperature was reduced to 50°C the theoretical efficiency would drop to just 9% and in practice most engines wouldn’t work at all.
A third use of heat is for processes. Heat is directly used in many industrial processes, for example to induce chemical reactions, melt substances and dry products. The temperature required depends on the specific process. Examples of processes requiring high temperatures include steelmaking (1,600°C), glass production (1,600°C), firing cement (1,375°C), brick baking (900-1,200°C), paper pulp processing (1,040°C) and aluminium production (960°C). Lower temperatures are required for chemical processes (90-220°C), industrial distillation, concentrating and drying (75-160°C), cooking, sterilising and bleaching (50-110°C), and agricultural space and media heating (0-110°C).
Steam boilers are often used to supply process heat for lower temperature processes. These may be direct or indirect, with direct steam heating injecting steam directly into the process and indirect steam circulating in through pipes. Indirect systems have a cold water feed which is suitable for pre-heating with relatively low temperature waste heat. Indirect systems, however, have the same issues with relatively high return temperatures as central heating systems.
CHALLENGES
There are two main challenges with utilising waste heat. Firstly, heat is rejected from a processes at a lower temperature than that same process can use it. For example, the main cause of inefficiency in heat engines is the heat they reject, and this is always at a lower temperature than they can use. Any waste heat must therefore be used in a different process, which requires heat at a lower temperature.
The second challenge is therefore to find suitable processes requiring this lower temperature heat. Moving heat over any significant distance requires capital-intensive equipment and will result in a further loss of both energy and temperature. The lower-temperature process must therefore be located relatively close to the source of the waste heat. Preheating can sometimes be a useful way to use low-grade heat in higher temperature processes. However, many processes already cycle a working fluid, meaning that the waste heat cannot further increase the feed.
These challenges mean it is impractical to recover waste heat from most small and lower temperature sources. The heat rejected from a process is almost always at a far lower temperature than the operating temperature of the process itself.
Where there are processes producing very large quantities of waste heat at high temperatures, there is far more potential to recover heat. It is then worth planning to locate different processes together so that one can use the waste heat from another. There are many established examples of this, some of which are below.
Combined cycle power plants use the waste heat from one heat engine to power a second heat engine which operates at a lower temperature. These typically involve a gas turbine with heat recovered from its exhaust used to power a steam turbine. While this gives higher efficiencies than other power stations, some heat is still rejected; it is still limited to less than Carnot efficiency.
Organic Rankine cycle engines use organic fluids that vaporise at a lower temperature than water, allowing lower-temperature heat to produce useful work, typically to generate electricity. The same Rankine cycle is used as a conventional steam turbine power station. The organic fluids used are the same hydrocarbons and F-gases used in refrigeration and heat pumps.
Combined heat and power generation systems use heat rejected from power generation for space heating in buildings. It is often combined with a district heating network that supplies hot water to buildings over an extended area.
Industrial process heat may also be used for district heating. Using both sides of a heat pump can work in many applications. Heat pumps essentially cause heat to flow up hill, from a low temperature area to a higher temperature area. This makes the cold side colder and the hot side hotter, the opposite of the levelling out entropy process that heat would do left to its own devices. In most current applications, only one side of a heat pump is deliberately used, such as to refrigerate a volume, or to heat water.
Kristian Strand, president of Danfoss Climate Solutions, says: “If you look at the global growth in data centres, they’re going to grow with a factor of 3.6 in the next decade. We need to focus on how we can make data centres actually contribute [energy] instead of just consuming energy. Today in a data centre you have to cool it down. The computers are generating heat, and in many instances today, we just blow out this heat to the surrounding air without getting anything out of it. The technology to use here is a heat pump. So you basically capture that heat, and then you bring up the temperature using a heat pump. Then you have energy that you can use, for instance, for district energy heating up houses, or for other processes.”
USING HEAT
High temperature processes supporting lower temperature ones can be the simplest way to use waste heat, if you are fortunate enough to have such processes in close proximity. When planning the location of different processes and facilities this could become a significant consideration and local governments may plan a role in zoning industrial areas to facilitate this.
The approach to planning and implementing waste heat utilisation depends very much on the details of your site, including stage of operations, size, scope and timescale. Consideration of how to use waste heat within an existing facility will be very different to considering an optimum location for a new facility to enable it to both utilise and supply waste heat streams from external entities. However, common steps include:
1. Identify potential sources of waste heat from your processes and from any relevant external entities. The most useful sources will be in the form of flows of exhaust gases and hot liquids. These should be quantified in terms of temperatures, flow rates and intermittency or dispatchability and location
2. Identify process heat requirements in terms of temperatures, flow rate, location and scope for demand side response. In this context, demand side response means turning the process up and down, on or off, depending on the availability of the waste heat stream
3. Evaluate the potential to generate energy from your waste heat streams
4. Evaluate where heat pumps might be utilised to more efficiently integrate heating and cooling requirements
5. Identify the potential uses for waste heat within your organisation, in terms of technical feasibility based on temperatures and flow rates
6. Carry out feasibility studies for each of the potential uses identified in step 4. This should consider the expected energy savings, CapEx, OpEx and ROI for each use case. Equipment that may need to be costed at this stage could include pipework, heat exchangers, heat pumps, and organic Rankine cycle systems
7. Downselect and implement the chosen uses for waste heat.
There are many opportunities for using waste heat. Some organisations may be able to directly use rejected heat from high temperature processes for lower temperature processes. In other cases the use of heat pumps and organic Rankine cycle generators may be required to use waste heat streams. The more widespread utilisation of waste heat sources could be facilitated by zoning of industries producing waste heat with users, together with infrastructure networks for district heating.