Industrial waste heat and waste heat recovery systems
By H. Saadaoui
Due to the increasing trend of rising fuel prices in recent decades and the ever-growing concern about global warming, companies are facing pressure to reduce greenhouse gas emissions and improve the energy efficiency of their installations.
Therefore, the use of waste heat recovery systems in the industrial waste heat processes was one of the main research fields aimed at improving energy efficiency and reducing harmful emissions.
Industrial waste heat is, by definition, the energy produced during industrial waste heat processes and that is not used in the process (wasted energy or released into the environment). Residual heat sources mainly consist of heat losses transferred by conduction, convection and radiation from the products, industrial process equipment and the heat released by combustion gases .
Heat losses can be classified into three categories :
- Losses at high temperature: for all losses at temperatures above 400°C;
- Losses at medium temperature: this category includes all losses between 100 and 400°C;
- Low temperature losses: for all losses at temperatures below 100°C.
Usually, most of the lost heat in the high temperature range comes from the direct combustion processes. Lost heat in the medium temperature range comes from the exhaust gases from the combustion units and the one in low temperature range comes from parts, products and equipment of the treatment units .
Waste heat recovery systems (WHR) are put in each category of loss in order to obtain an optimal recovery efficiency.
The following figure gives an overview of the heat losses in different industrial sectors and their recycling into electrical energy .
WHR sytems applicable for the medium temperature category: case of losses by exhaust gases
Three different technologies can be selected to use the exhaust gas heat (losses at medium temperatures):
- Heat exchanger used in the hot water supply system
- Thermal cooler used in the cold supply system
- Rankine Cycle for the production of electricity.
Heat exchanger used in a hot water supply system
The figure below shows the heat supply system using a central heat exchanger (H-Ex). Exhaust gases enter the heat exchanger at Te,g and exit at Ts,g. Their heat is used to produce hot water which will go through the heat supply system between the high temperature storage tank (HTS), the heat user and the low temperature storage tank (LTS). HTS and LTS are storage tanks that can be used if the released heat isn’t available when the system is running. The HTS hot water at Tw_H is delivered to the user and then goes back to the LTS at Tw_L where it is collected and pumped through the heat exchanger if necessary. Therefore, there are two hot water loops. The inner loop includes LTS, pump, H-Ex and HTS. The outer loop includes HTS, pump, heat distribution unit (HDU), pipeline, heat user and LTS. Each loop works individually depending on the function of the operating program and on the storage capacity of the two tanks.
The figures below show the block diagram of a heat actuated cooling system. The released heating sources are high temperature exhaust gases that heat the cooler’s generator and go through Tg,in to Tg,out. The refrigerator’s water is cooled down in the evaporator from Tw,H to Tw,L and goes through the cold supply system between the low temperature cooling tank (LTS), the cold user and the high temperature cooling tank (HTS). LTS and HTS can be used to create an accumulation of cold air that can be used when the released heat is temporarily unavailable. The LTS cold water at Tw,L is pumped towards the user and then, goes back to the HTS at Tw,H where it is collected and pumped into the refrigerator if necessary.
There are two cold water loops. The inner loop is composed of HTS, refrigerator, and LTS. The outer loop is composed of LTS, refrigerator and HTS. Each loop operates individually regarding its operating program and the storage capacity of the two cold storage tanks. The heat extracted from the cooling water of the cold supply system is sent to the cooler inside the evaporator and must be released into the environment. Then, the cooler’s vapour is compressed to a high-pressure level using the compression-absorption method. The vapour’s condensation, on account of pressure, allows the release of heat into the environment. This final stage of the process takes place inside the condenser and the refrigerator’s absorber which are cooled down by air or cooling water from the cooling tower where the transfer of the remaining heat to the environment takes place.
Rankine Cycle for electricity production
The figure below shows the Rankine Cycle. The figure after that shows the operating principle. Exhaust gases at Tg,in exchange their heat with the superheater, evaporator and preheater. Then, they are released into the atmosphere at 120°C. That temperature is set as lower limit to avoid any condensation in the system. The high pressure working fluid’s vapour expands in the turbine and enters the regenerator (state 2), where the released vapour rejects heat to the vapour cooler (VC) integrated in the condenser cooled down by water, where it eventually condenses in the liquid phase (state 4). The condensate is then pumped to operating pressure (state 5) and sent to the heat exchanger system to produce fresh superheated steam at high pressure (state 1).
Three fluids commonly adopted for technical applications have been chosen to be applied as a possible working fluid: benzene, R11 and R134a. The choice of working fluid depends on the temperature range of the heat source and the selection criteria. The selection criterion used is the cost of producing electricity (EUR / kWh) and not the efficiency of the system, which may require high investments and deteriorate economic viability. This means that the cost of electricity production (CEP) is first calculated for the three working fluids and those with the lowest CEP are selected.
 I. Johnson, W. T. Choate and A. Davidson, “Waste Heat Recovery. Technology and Opportunities in U.S. Industry” BCS, Inc., Washington, D.C., 2008.
 S. Bruckner, S. Liu, M. Laia, M. Radspieler, L. F. Cabeza and L. Eberhard, “Industrial waste heat recovery technologies: An economic analysis of heat transformation technologies” Applied Energy, vol. 151, no. 1, pp. 157-167, 2015.
 K. Zeb, S.M.Ali, B.Khan, C.A.Mehmood, N.Tareen, W.Din, U.Farid, A.Haider, “A survey on waste heat recovery: Electric power generation and potential prospects within Pakistan”, Renewable and Sustainable Energy Reviews, Volume 75, August 2017, pp.1142-1155.