Waste heat to power (WHP) is the process of capturing waste heat discarded by an existing industrial process and using that heat to generate electricity, with no additional fuel, combustion or emissions. Whenever an industrial process, such as those that occur in refineries, chemical plants, steel mills and general manufacturing, transforms raw materials into useful products, heat is generated as a byproduct.
This heat represents a source of energy that is produced whenever the operation is running. If not recovered for reuse as process heat or to generate power, it will dissipate into the atmosphere as a wasted opportunity.
Combined Heat and Power
The use of a single fuel source to generate both thermal energy and electricity is referred to as combined heat and power (CHP). The recovery of industrial waste heat for power (WHP) is one type of CHP.
The CHP process usually includes a prime mover, a generator, a heat recovery system and electrical interconnection equipment configured into an integrated system. It is a form of distributed power generation, located at or near the energy-consuming facility. The energy produced offsets some of the energy that would otherwise have to be purchased from the primary source.
Figure 1: Combined Heat and Power
In addition, CHP-generated electricity has no transmission losses that occur with the delivery of electricity from the central station power plant. The overall effect of using less primary energy is lower greenhouse gas (GHG) emissions.
A topping cycle is the most common type of CHP, where a heat engine is used to generate power, and the waste heat from the power generation equipment is then recovered to provide useful thermal energy. Figure 1 shows a coal-fired boiler creating high-pressure steam that drives a turbine generating electricity. Useful thermal energy is captured from the low-pressure steam turbine exhaust and sent to an industrial process or district heating system.
Electricity generated from waste heat has the potential to displace power from sources that generate emissions. Every MWh of electricity generated from waste heat replaces 1,100 lbs. of CO₂ equivalent from a new coal-fired power plant or 1,000 lbs. of CO₂ equivalent from a natural gas combined cycle plant. If just one-third of the U.S. WHP potential was realized, it could offset up to 20.5 million tons of CO₂ that are released into the atmosphere.
The Opportunity for WHP
The energy-saving potential for WHP in the United States is extremely difficult to calculate. Estimates based on studies conducted by the U.S. Department of Energy (DOE) 2008 titled, “Waste Heat Recovery: Technologies and Opportunities in U.S. Industry”, and a May 2005 report published by the Oak Ridge National Laboratory titled, “Characterization of the U.S. Industrial/Commercial Boiler Population,” are shown in Table 1. The work potential available from waste heat generated in the industrial and commercial sectors may be more than 1,050 TBtu/year.
However, not all this work potential can be used to generate electricity, since some waste heat is unsuitable for capture and reuse. From the waste heat that is captured and reused, only a portion can be used to generate electricity; the rest is used directly as heat.
As a comparison to other sources, in 2009, approximately 697TBtu of energy was generated by wind and 3,900TBtu of energy was generated by biomass fuel.
Additional sources of waste heat exist, but the estimates in Table 1 include the sources that are most likely to be used for power generation. Even within these sources, there is variability in the quality of the waste heat that impacts its suitability for power generation. The three most important variables are temperature, flow rate and the cleanliness of the waste heat. According to the 2008 DOE report, much of the existing heat recovery that is currently carried out in the United States utilizes clean, high-temperature waste heat sources in large capacity systems.
Generating power efficiently from waste heat recovery is heavily dependent on the temperature of the waste heat source. In general, economically justifiable power generation from waste heat has been limited primarily to medium and high-temperature waste heat sources (> 500° F). Emerging technologies, such as organic Rankine cycles, are beginning to lower this limit, and further advances in alternative power cycles will eventually enable economic feasibility of generation at even lower temperatures.
Figure 2: Steam Rankine Cycle
Steam Rankine Cycle (SRC): In the steam Rankine cycle, the working fluid, which is water, is first pumped to an elevated pressure before entering a heat recovery boiler. The pressurized water is vaporized by the hot exhaust and then expanded to lower temperature and pressure in a turbine, generating mechanical power.
The low-pressure steam is then exhausted to a condenser at vacuum conditions, where heat is removed by condensing the vapor back into a liquid. The condensate from the condenser is then returned to the pump, and the cycle continues.
The principal limitation of water as the base fluid in an engine is the high boiling point of water, requiring a relatively high heat level to accomplish the phase change. Therefore, if a heat exchanger is used to collect process waste heat, the temperature of the heat must be significantly above the boiling point of water.
Organic Rankine Cycles (ORC): ORCs use an organic working fluid that has a lower boiling point, higher vapor pressure, higher molecular mass and higher mass flow than water. These features enable higher turbine efficiencies than in an SRC.
ORC systems can be utilized for waste heat sources as low as 300° F. They have been used to generate power in geothermal power plants, industrial process fluids and, more recently, in pipeline compressor heat recovery applications. The thermal efficiency of the ORC is often quite low, but when it is used in combination with a thermal stream that would otherwise be wasted, the process may be economically justified.
The Kalina Cycle: A Rankine cycle that uses a mixture of water and ammonia as the working fluid allowing a more efficient energy extraction from the heat source. The Kalina cycle functions with waste heat temperatures of 200° F to 1000° F. It is 15% to 25% more efficient than ORCs at the same temperature. Kalina cycle systems are frequently used overseas in geothermal power plants, where the hot fluid is very often below 300° F.
The three types of Rankine power cycles have some characteristics in common. However, there are advantages to each. They include:
- SRCs are the most familiar to industry and are economically preferable when the source heat temperature exceeds 500° F.
- ORC and Kalina cycle systems are generally used for temperatures below 500°F and are more efficient in moderate temperature ranges.
- Kalina systems have the highest theoretical efficiencies. They are generally better suited for large power systems of several megawatts or greater.
Several other advanced technologies that can generate electricity directly from heat are in the research and development stage, and they could eventually provide additional options. These technologies include piezoelectric, thermionic, thermo-photovoltaic (thermo-PV) and thermoelectric devices, which are small solid-state devices that generate electricity directly from heat.
Steam Rankine Cycle
The Oxbow Corporation coke plant in Port Arthur, Texas can produce up to 700,000 short tons per year of calcined petroleum coke, used primarily in aluminum production.
Port Arthur Steam Energy LP (PASE) is a Waste Heat to Power (WHP) plant that uses otherwise wasted heat from the coke production facility and converts it into five megawatts of electricity consumed on site and sold to the grid. In addition, steam is sold to the neighboring Valero petroleum refinery for crude oil processing.
PASE earned the EPA’s 2010 Energy Star Award for an emission-free system that recovers heat from the flue gas and converts it into electricity and steam.
Organic Rankine Cycle
Along the Alliance Pipeline, NRGreen installed four 5-MW OEC plants to capture heat from compressors pushing gas along the 2,331-mile stretch from British Columbia to Chicago. Each plant can yield 5 MW of power that NRGreen sells to the provincial grid.
The benefits of heat recovery to NRGreen include:
- Fully automated and nearly unattended operation provides each plant with the built-in ability to synchronize with the grid and even do load-following.
- Environmental friendliness promises to earn green credit vouchers or other incentives and rewards.
- At 5-MW output each, they fit into most distribution systems and typically won’t tie up heavy-gauge transmission capacity.
- The pipeline’s turbines are running at a 99.4% theoretical availability rate, which is excellent for using their continuous-flow baseload type waste heat.
The Canoga Park large scale power plant was the first to be commissioned using the Kalina Cycle. It was built with U.S. Department of Energy (DoE) support to demonstrate the technology.
Initially, the energy source was waste heat from a nuclear steam generator test plant. Later the energy source was changed to the exhaust heat from a gas turbine. The unit was in operation for more than five years (1992-1997) with good reliability and accumulated about 10,000 operational hours.
The turbine throttle fluid conditions were 515°C and pressure of 110 bars and the generating capacity of the bottoming cycle was about 3MW with surplus power being sold to the local utility. The Canoga Park combined cycle power plant demonstration was rated at about 6.5 MW.
Current Market Status
There are currently 36 WHP projects in the U.S. totaling 575 MW of power generation capacity, as shown in Table 2. Most of these industrial WHP systems use a heat recovery boiler, steam turbine and generator, which are limited to waste streams with relatively high temperatures (> 500 ° F). Other systems use lower temperatures and smaller sizes, include ORCs, ammonia-water systems and thermo-electric generators with solid state systems that require no moving parts and sit directly in the waste stream.
Table 2: Existing WHP Projects in the United States by Industry (2012)
Utilizing liquid streams below 200° F and gas streams below 500° F for conversion to electricity typically remains economically impractical with today’s technologies. WHP is generally considered only when the waste heat cannot be used directly within the process, or other recovery methods are not practical within the facility. While the costs of these systems currently remain high, and commercial demonstration is limited, the technologies continue to evolve rapidly.
In the United States recently, waste heat to power projects have been growing slowly. Electricity from waste heat applications must be extraordinarily cheap for it to compete effectively with low electricity prices from utilities. A marginal cost-benefit plus the risk that the technology will interfere with industrial processes in many cases is sufficient to shelve a project.
Three factors could accelerate the implementation of WHP systems in the coming years: increased government incentives, new business models and recent technological developments.
Although power from waste heat generates clean energy with no emissions, it is often considered an energy efficiency improvement rather than a renewable energy implementation. Regardless of classification, industry members feel the technology warrants government incentives like those enjoyed by other clean energy technologies. The lack of extensive state or federal incentives to promote waste heat to power generation has been a drag on industry growth.
New Business Models Reduce Barriers to Entry
Industrial companies considering implementing a WHP system run the risk that application of the project will negatively impact their operations and that even a successfully implemented project will fail to deliver anticipated returns.
New business models are emerging in the industry that help justify those risks. The return on investment in waste heat to power projects is dependent on the cost of the electricity the project will be displacing. For industrial companies, this cost can be quite low, which reduces the incentive to generate their own electricity.
Several companies are focused on two possible opportunities. One area is the enhancement of the Kalina cycle, an alternative to the organic Rankine cycle that utilizes a mixture of water and ammonia as the working fluid. The other area involves the development of new expanders for the organic Rankine cycle, with an emphasis on economic electricity production in small waste heat to power projects operating at low temperatures.
Waste heat to power (WHP) has an enormous potential to capture an untapped clean and reliable power source. While some obstacles remain to implementation, they are far from insurmountable.
The technology is proven and has been demonstrated. Only the economics, including government incentives, remain to incentivize industry investment.