Efficiency, waste heat and recovery: Waste heat by temperature grade
Understanding Energy / Module 12: Efficiency, waste heat and recovery Part 4 of 5Waste heat by temperature grade
Waste heat is not one thing. A stream of 900-degree furnace gas and a pond of 35-degree cooling water both count as losses, but they are worth very different amounts. Temperature decides what a stream can be used for and how much it costs to capture. This part maps where waste heat comes from and how to match it to a use.
4.1 Industry throws away a fifth to a half of its energy, mostly in exhaust and cooling water
The US Department of Energy estimates that 20 to 50% of the energy that goes into industry is lost as waste heat. It leaves in four main ways. Hot exhaust gases leave furnaces, kilns, boilers, ovens and engines through stacks. Cooling water carries heat away from compressors, engines, condensers and chemical reactors. Heat leaks through the walls of hot equipment. And heated products, such as hot steel slabs, glass or clinker, carry heat out of the plant before they cool.
A 2008 study for the department estimated that US industry leaves somewhere between 5 and 13 quadrillion British thermal units a year unrecovered, roughly 5 to 14 exajoules, comparable to the whole energy use of a large European country. Exhaust gases from the processes the study examined alone accounted for about 1.6 EJ.
The study sorted waste heat into three temperature grades. High-grade heat, above about 650 degrees Celsius, comes from steel furnaces, glass melting, coke ovens, aluminium furnaces and some metal refining. Medium-grade heat, between about 230 and 650 degrees, comes from boiler and gas turbine exhausts, engine exhausts, heat-treating furnaces, drying and baking ovens and cement kilns. Low-grade heat, below about 230 degrees, comes from gas leaving boilers after their first heat recovery, cooling water from compressors and engines, air-conditioning condensers and hot liquids and solids.
Most unrecovered heat is in the bottom grade. The study found that about 60% of unrecovered industrial waste heat was low quality, below 230 degrees. At global scale, Forman and colleagues found that 63% of waste heat streams across all sectors were below 100 degrees. The easy, high-grade heat has often been captured already. What remains is plentiful but harder to use.
- Industrial waste heat
- High grade, above about 650 C
- Steel furnaces and coke ovens
- Glass melting furnaces
- Aluminium furnaces
- Medium grade, about 230 to 650 C
- Boiler and gas turbine exhaust
- Engine exhaust
- Cement kilns, drying and baking ovens
- Low grade, below about 230 C
- Boiler flue gas after first recovery
- Cooling water from compressors and engines
- Air-conditioning condensers
- Hot process liquids and solids
- High grade, above about 650 C
- BCS Inc. for US Department of Energy, Waste Heat Recovery: Technology and Opportunities in U.S. Industry (2008) · web
- US Department of Energy, Waste Heat Recovery Basics · web
- Forman, C., Muritala, I. K., Pardemann, R. and Meyer, B., 'Estimating the global waste heat potential', Renewable and Sustainable Energy Reviews 57 (2016) 1568-1579 · web
4.2 Temperature sets how much of the heat can become work
The second law turns temperature into money. Carnot's formula gives the most work that heat at a given temperature can provide when rejected to the surroundings. With surroundings at 25 degrees Celsius, heat at 100 degrees could at best give about 20% of its energy as work. At 200 degrees, about 37%. At 400 degrees, about 56%. At 650 degrees, about 68%. Real machines achieve perhaps half to two-thirds of these limits.
Temperature also decides what the heat can do directly. Heat at 900 degrees can preheat furnace air, raise high-pressure steam or drive a chemical reaction. Heat at 300 degrees can make medium-pressure process steam. Heat at 120 degrees can dry grain, distil liquids or drive an absorption chiller. Heat at 50 degrees can warm buildings or greenhouses, and little else.
A second factor is the medium. Clean, dry gas is easy to pass through a heat exchanger. Dirty gas full of dust, sulphur or alkalis, common in cement, steel and glass plants, fouls and corrodes heat-transfer surfaces. Flue gas cooled below its dew point condenses acidic water, which can eat through steel. These practical issues explain why the US study described low-temperature recovery as often uneconomic: it needs large heat-exchange areas, corrosion-resistant materials, and a user for the heat.
The upshot is a hierarchy of value. High-grade heat should rarely be wasted; it can often pay for itself within a year or two. Medium-grade heat suits steam raising, power generation and process heating. Low-grade heat is abundant but is worth most when used close by, at the right time, or upgraded with a heat pump.
| Temperature | Carnot limit (25 C sink) | Typical best use |
|---|---|---|
| Above 650 C | over 68% | preheat combustion air or charge; raise high-pressure steam; generate power |
| 230 to 650 C | about 41 to 68% | process steam; steam Rankine or organic Rankine power; feedwater heating |
| 100 to 230 C | about 20 to 40% | drying, distillation, absorption cooling; organic Rankine power at the upper end |
| Below 100 C | under 20% | space and water heating, greenhouses, district heat; heat pump upgrading |
- BCS Inc. for US Department of Energy, Waste Heat Recovery: Technology and Opportunities in U.S. Industry (2008) · web
- Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M1 'Conservation and efficiency'
- Forman, C., Muritala, I. K., Pardemann, R. and Meyer, B., 'Estimating the global waste heat potential', Renewable and Sustainable Energy Reviews 57 (2016) 1568-1579 · web
4.3 Recovery works when a source meets a sink: same place, same time, right temperature
Waste heat is only valuable if something nearby needs heat at a lower temperature at the same time. Engineers call the hot stream a source and the user a sink. Most failed recovery projects fail on one of three mismatches.
Place is the first. Heat is expensive to move. Hot water pipes lose heat and cost a lot to lay, so a source and a sink usually need to be within a few kilometres, and preferably on the same site. A steel works next to a town can supply district heat; a remote cement plant cannot.
Time is the second. A bakery's ovens may run by day while the nearby housing needs heat at night. A summer peak in waste heat may meet a winter peak in demand. Heat storage, in hot water tanks or underground, can bridge hours or seasons, at a cost.
Temperature is the third. The sink must need heat cooler than the source, with a margin for the heat exchanger. Cooling water at 35 degrees cannot heat a building's radiators at 70 degrees without a heat pump.
Within a single plant, the method for getting this right is called pinch analysis. Engineers list every stream that needs heating and every stream that needs cooling, plot them by temperature, and find the arrangement of heat exchangers that recovers the most heat. It often shows that a refinery, chemical plant or food factory can cut its fuel use substantially simply by rearranging how existing streams exchange heat. Natural Resources Canada's guide reports that studies of food and drink plants typically find savings of 15 to 40% of purchased fuel, about 5% of it from quick wins that pay back within a year.
The principle is a cascade. Use heat at the highest temperature first, then pass what is left to the next use down, and so on, until it reaches the surroundings.
preheats furnace combustion air, cutting fuel use
2 Waste-heat boilerraises steam at a few hundred degrees for process use or power
3 Economiserpreheats boiler feedwater with gas at about 200 C
4 Dryer or hot water loopuses gas or water at 60 to 120 C for drying, washing or space heat
Outputs Stack gasabout 100 C or below Fuel savedoften a fifth or more of site fuelEach step takes heat at a lower temperature than the one before; illustrative values.
Source: BCS Inc. for US Department of Energy, Waste Heat Recovery: Technology and Opportunities in U.S. Industry (2008), https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf (5 to 13 quads unrecovered; about 1,500 TBtu from the exhaust streams analysed; 60% below 450 F (232 C); grades at 450 F and 1,200 F; payback well under 3 years; heat pump costs taken from IEA CADDET 1997; a dated 2008 study)See it in the table
H₂O WaterAl AluminiumSt SteelCem CementCheck your understanding
According to the US DOE study, roughly what share of unrecovered industrial waste heat is low quality, below about 230 degrees Celsius?
About 60% of unrecovered heat is low grade. High-grade heat has often already been recovered, because it pays back quickly.At most, what share of heat at 100 degrees Celsius can become work when the surroundings are at 25 degrees?
Carnot's limit is 1 minus 298/373, about 20%. Real machines achieve roughly half of that.Which is the most common reason a waste heat recovery project fails?
Place, time and temperature must match. Without a nearby sink, recovered heat has nowhere to go.