Heat recovery at power generating stations
Background
Globally, thermal power plants achieved a conversion efficiency of 36% in 2011. So for every 100 parts fuel that went into the generator, they provided 36 parts useful energy. This is true for electricity generation stations large and small and almost all of the inefficiency (about 60%) is lost thermal energy, or heat, which is a natural byproduct of electricity generation
By contrast, co‐generation units (which utilize the electricity and the heat from the generator) converted about 58% of energy input into electricity and useful heat in the same year. State‐of‐the‐art co‐generation units today reach conversion efficiencies of as much as 90% (IEA)
By using heat and power simultaneously, Cogeneration or Combined Heat and Power (CHP), has significant environmental and financial benefits, including:
- Power plant heat can be used for many things: it can generate warming (i.e. heating a hospital), cooling (i.e. air conditioning a mall), water purification (through boiling the water), and even create more electricity (through combined cycle systems).
- Avoid the cost of capital equipment. In some cases, cogeneration systems can nullify the need for a boiler, air conditioner, etc. resulting in cost savings
- Save money on energy bills through reduced fuel consumption (displacing burning fuel in a boiler) or and/or reduced electricity consumption (not having to run an air conditioner).
- Reduce greenhouse gases. Using the heat produces no incremental emissions and reduced fuel or electricity use translates into reduced greenhouse gases. When cogeneration takes a project from 36% efficient to 90%, the avoided greenhouse gases, and other pollutants can be significant.
- Increase energy security. By having an electric generator and provider of heat on-site, a facility owner takes into their own control the power that is delivered to their site.
Despite these benefits, only 9% of global electricity generation uses co‐generation technologies.While some countries have achieved a high share of co‐generation in electricity production (for instance, Denmark has more than 60% and Finland almost 40%), most countries have not been that successful. (IEA) Herein lies a huge opportunity for retrofitting existing generator stations to convert them into cogeneration units.
Many of the new generators being installed are including forms of heat recovery at their sites. At universities and city districts, generator heat is being used to warm all the nearby buildings. In warm areas like Southeast Asia, generator heat is used for cooling to provide air conditioning to malls. And many of the new large scale gas turbines are installed as combined cycle plants, where heat is converted into additional electricity, increasing the overall site to 50-60% efficient. But more could be done with new plants, and many of the existing sites that have no heat utilization can be retrofitted for combined heat and power and made much more efficient.
Sources of heat
The conversion efficiencies and percentages of heat available are roughly the same regardless of the fuel-based power generator being used (turbine, reciprocating engine fuel cell, etc.). However, different power generation technologies release heat in different ways, which is important to take into consideration. All of these technologies will have a specification sheet that indicates exhaust temperatures and flow rates, which can be plugged into our calculator to estimate the quantity of usable heat. At the end of this article, we have listed some nuances and tips about each of the heat sources.
Making a project happen
One of the nice things about many cogeneration projects is that the prime mover (generators) often run all day and night. This means that the heat source can be available all day and night to supply heat. There are many great tools available for understanding how projects come together and the associated economics including our own guides. The project economics typically hinge on the following variables:
- Price of fuel to run the generator
- Cost to produce the heating (fuel cost, OpEx, CapEx for a boiler)
- Useful heating hours
- Value of the electricity produced (used on-site or sold to the grid)
- Any incentives and/or demand charges that it offsets
Understandingchp.com has put together a useful calculator to estimate the economics for CHP projects when the heat used displaces using a natural gas boiler.
Common sources of heat
Reciprocating engines
Reciprocating engines are reliably used across the world to generate electricity on a variety of fuels (natural gas, biogas, landfill gas, diesel, etc.). They range in size from the one in your car to ones that take up entire buildings.
Approximately 60-70% of the total energy input to the engine is converted to heat that can be recovered from the engine exhaust and jacket coolant, while smaller amounts are also available from the lube oil cooler and the turbocharger's intercooler and aftercooler (if so equipped). Steam or hot water can be generated from recovered heat that is typically used for space heating, reheat, domestic hot water and absorption cooling.
Heat in the engine jacket coolant accounts for up to 30% of the energy input and is capable of producing 200°F hot water. Some engines, such as those with high pressure or ebullient cooling systems, can operate with water jacket temperatures up to 265°F. Engine exhaust heat is 10-30% of the fuel input energy. Exhaust temperatures of 850°- 1200°F are typical with lower temperatures often found in low and medium speed engines, and higher temperatures in higher speed engines. Only a portion of the exhaust heat can be recovered since exhaust gas temperatures are generally kept above condensation thresholds. Most heat recovery units are designed for a 300°-350°F exhaust outlet temperature to avoid the corrosive effects of condensation in the exhaust piping. The minimum temperature can vary depending on the cleanliness of the fuel the engine consumes. (US DOE)
An additional potential benefit arises when using the engine's jacket water heat, which normally requires radiator fans to cool it. By using some or all of the heat from the jacket water, the electric load from the radiator fans can be reduced, resulting in cost savings from lower electricity bills.
Steam turbines
Steam turbines are one of the most versatile and oldest prime mover technologies used to drive a generator or mechanical machinery. Steam turbines are widely used for CHP applications in the U.S. and Europe where special designs have been developed to maximize efficient steam utilization. The capacity of steam turbines can range from a fractional horsepower to more than 1,300 MW for large utility power plants.
The thermodynamic cycle for the steam turbine is the Rankine cycle. The cycle is the basis for conventional power generating stations and consists of a heat source (boiler) that converts water to high pressure steam. The steam flows through the turbine to produce power. The steam exiting the turbine is condensed and returned to the boiler to repeat the process.
Heat recovery methods from a steam turbine use exhaust or extraction steam. The amount and quality of the recovered heat is a function of the entering steam conditions and the design of the steam turbine. Exhaust steam from the turbine can be used directly in a process or for district heating. Or it can be converted to other forms of thermal energy including hot water or chilled water. Steam discharged or extracted from a steam turbine can be used in an absorption chiller. (US DOE)
Gas turbines
Gas turbines typically have high temperature exhaust in the 500-700C (932-1292F) range and represent 60% of the energy input that goes into the turbine. This high quantity, high temperature heat makes turbines excellent CHP candidates. These high temperature exhaust is why so many systems use heat recovery steam generators to power a heat recovery steam generator (HRSG) that can drive a turbine generator or provide steam to a district heating network. Gas turbines can range in output size from 10MW to 500+ MW of electrical output. GE has put together a good list where you can see the specs of the exhaust profiles and output for a bunch of different sizes.
The simple cycle gas turbine is the least efficient arrangement since there is no recovery of heat in the exhaust gas. Hot exhaust gas can be used directly in a process or by adding a heat recovery steam generator (HRSG), exhaust heat can generate steam or hot water. Gas turbines are frequently used in district steam heating systems since their high quality thermal output can be used for most medium pressure steam systems. (US DOE)
For larger gas turbine installations, combined cycles become economical, achieving approximately 60% electric generation efficiencies using the most advanced utility-class gas turbines. In a combined cycle setup, the exhaust gas heat is used to convert water to steam that is passed through a steam turbine, which adds additional electrical capacity for the same amount of fuel. Organic Rankine Cycle units can also be used with gas turbines to create a combined cycle. Although they are generally less efficient that steam cycle units, they are more adaptable to load changes with the gas turbine and generally do not require makeup water. GE has a whole line of products dedicated to the gas turbine + ORC arrangement
Microturbines
Microturbines are essentially small versions of the gas turbines. Several manufacturers are have competing turbines in the 25-250 kW range, however, multiple units can be integrated to produce higher electrical output while providing additional reliability.
Microturbines are typically less efficient than the larger aeroderivative gas turbines. Typical electrical efficiencies are 30%, meaning 65% of energy lost is in the form of heat in the exhaust. Unfortunately, however, the exhaust heat is around 280C, which is a relatively low temperature and means that only a portion of the exhaust energy can actually be captured. At this low of temperature, one can create low pressure steam or hot water systems for CHP, and organic rankine systems for combined cycle applications are most common forms of heat recovery. Example Capstone Microturbine spec sheet
Fuel Cells
Although their use isn't as widespread as engines or gas turbines, fuel cells offer the potential for clean, quiet, and very efficient power generation, benefits that have driven their development in the past two decades. Fuel cells offer the ability to operate at electrical efficiencies of 40-60% and up to 85% in CHP applications.
However, exhaust temperature varies significantly by the conversion process in the fuel cell. As you can see from the chart below, exhaust temperatures can range from 150C to 1850C and efficiencies range from 25% to 55%. (Understanding CHP)
Like any other generator, it’s important to check the spec sheet of the equipment you have to determine the quantity of heat available.
Examples
All of these can be found in this great IEA Report. Scroll down to page 27 for great details on each one.
- The Eresma Cogen project consists of a gas engine‐based 13 megawatts electric (MWe) co‐generation system that supplies electricity and heat to a distillery factory in Segovia, Spain. The generation system provides 70% of process steam and all the electricity requirements of the industrial site, and it exports the excess electricity to the grid, saving roughly 16 kilotonnes (kt) of carbon dioxide (CO2) every year.
- The co‐generation plant located in the gas processing complex of Nuevo Pemex in Tabasco, Mexico provides heat and power for on‐site requirements and exports electricity to other users. The generation system has a 300 MWe installed capacity and includes two natural gas turbo generators with heat recovery equipment that result in 430 kt CO2 per year savings compared to conventional generation technologies.
- The Markinch biomass project consists of a 60 MWe co‐generation plant at the Tullis Russel paper mill in Fife, Scotland. The generation unit provides heat and electricity to support the paper production process, and it exports excess electricity to the grid. It is estimated that the plant will avoid 250 kt CO2 per year.
- The Sunstore 4 project is a district heating plant located in Marstal, Denmark that was developed to demonstrate the production of 100% renewable‐based district heating and flexible management of different intermittent energy sources with the assistance of thermal storage. The plant combines solar thermal, a biomass boiler coupled with an Organic Rankine Cycle (ORC), a heat pump and thermal storage. It is estimated to save 10.5 kt CO2 annually.
- The Bercy cooling plant is a district cooling facility in Paris, France with a current capacity of 44 megawatts (MWth). Free cooling assistance has been applied to this system resulting in a 34% increase of the average coefficient of performance (COP) of the plant’s chillers. Overall, the plant is estimated to save 7.4 kt CO2 annually.
- The solar thermal district heating system installed in the Princess Noura Bint Adbul Al Rahman University for Women (PNUW) in Ridyadh, Saudi Arabia is the world’s biggest operating solar heating project with 36 610 m2 of rooftop flat‐plate collectors. The system provides space heating and hot water to the university students and saves 5 kt CO2 per year.
Resources
- Assessment of CHP technologies - US Department of Energy [link]
- Linking Heat and Electricity Systems - International Energy Agency [link]
- Review on Exhaust Gas Heat Recovery for I.C. Engine - International Journal of Engineering [link]
- Recovery of Exhaust Heat from Gas Turbines - University L'Aquila, Italy [link]