Waste Heat to Energy (WtE) Solutions
Waste heat to Power (WHP) or Waste heat to Energy (WtE), terms that are many times used interchangeably with both being defined as the process of capturing heat being discarded by an existing industrial process in either a liquid or gaseous form and using that heat to generate power both in terms of electrical (kWe) and thermal (kWt). Energy intensive industrial processes—such as those occurring at refineries, steel mills, glass furnaces, and cement kilns—all release hot waste streams that can be harnessed with technologies to generate electricity. The recovery of industrial waste heat for power is largely an under-utilized type of combined heat and power (CHP), which is the use of a single fuel source to generate both thermal energy (heating ans/or cooling) and electricity.
The key advantage of WHP systems is that they utilize heat from existing thermal processes, which would otherwise be wasted.
The key components for determining the economic feasibility for power generation from waste heat sources would include:
- Is the waste heat source a gas or a liquid stream
- The availability of the waste heat—is it continuous, cyclic, or intermittent
- The load factor of the waste heat source—are the annual operating hours sufficient to amortize the capital costs of the WHP system
- Whether the temperature of the waste stream varies over time
- Is the waste stream a positive or negative pressure, and does this vary
- The composition of the waste stream; i.e. dirty, high viscosity
- Whether any contaminants are present that may corrode or erode the heat recovery equipment
Steam Rankine Cycle (SRC) – The most commonly used and efficient system for power generation from waste heat involves using the heat to generate steam in a waste heat steam generator, which then drives a steam turbine (ST). Typically requires waste heat greater than 500°F / 260°C.
Organic Rankine Cycles (ORC) – Other working fluids such as refrigerants, with greater efficiencies at lower heat source temperatures, are used in ORC heat engines. ORCs use an organic working fluid that has a lower boiling point, higher vapor pressure, higher molecular mass, and higher mass flow compared to water. Together, these features enable higher turbine efficiencies than in an SRC. Most ORC systems can be utilized for waste heat sources as low as 300°F /150°C. Technology has now paved the way for a smaller number of ORC systems at 170°C/ 77°F.
The Kalina Cycle is another Rankine cycle, using a mixture of water and ammonia as the working fluid, which allows for a more efficient energy extraction from the heat source. The Kalina cycle has an operating temperature range that can accept waste heat at temperatures of 200°F / 93°C to 1,000°F / 538°C and is 15 to 25 percent more efficient than ORCs of the same temperature level. Kalina cycle systems are becoming increasingly more popular overseas in geothermal power plants, where the hot fluid is very often below 300°F / 150°C. Typically, a more complex operation making them better suited for larger power generation facilities.