Methods of using waste heat for useful purposes
Traditional electricity generation plants burn coal or natural gas in a boiler to produce steam. The steam drives a turbine which spins a generator to make electricity. Modern power plants are usually able to convert 30-50% of the energy in the fuel they burn into electricity. The remaining energy is discarded as heat, usually through water or cooling towers.
Cogeneration is the simultaneous production of electricity and usable heat. The idea is to take the waste heat from a traditional power plant and put it to use. For this reason, cogeneration is often called Combined Heat and Power, CHP. By utilizing the waste heat from traditional power plants, overall efficiencies of cogeneration plants can reach 80-90%.
CHP is well suited for cold climates where buildings and houses have a high heating demand. The thermal energy from CHP plants can be used for domestic hot water, space heating, pool and spa heating, laundry processes, car washes, etc. CHP offers huge benefits to any facility that uses lots of thermal energy, such as apartment buildings, condominiums, schools, universities, assisted living facilities, hospitals, hotels, athletic clubs, laundries, car washes, and industrial and waste treatment facilities.
Trigeneration takes CHP one step further and uses the waste heat to drive an absorption chiller in order to provide electricity, heat, and cooling from a single plant. This broadens the range of applications and climates where trigeneration can be used. These plants can produce electricity and both heating and cooling simultaneously, which offers the ability to service space cooling and domestic hot water needs to facilities at the same time, such as during summer months or in warm climates.
Multigeneration systems are plants that produce more than three useful products. In addition to electricity, multigeneration plants may produce heating, hot water, cooling, water purification, gas liquefaction, hydrogen, oxygen, kiln drying, or other useful products. Any of these processes are driven by the electricity and heat produced. For example, natural gas is burned to spin a steam turbine which makes electricity and waste heat. Some of the electricity can power an electrolyzer that splits water into both oxygen and hydrogen gas. Some of the waste heat can drive a quadruple effect absorption system (QEAS) chiller that pre-cools the hydrogen and oxygen. More of the electricity can then drive the Linde-Hampson cycle to liquefy the hydrogen and oxygen - products that can be used for industrial and medical purposes. The waste heat from the QEAS can heat water for buildings and dry lumber. In this example, natural gas is the input, and electricity, space heating, hot water, dry lumber, liquified hydrogen, and liquified oxygen are all the products. It's worth noting that more products don't always lead to higher overall plant efficiencies since additional processes will take energy from others if the overall system efficiency is already high. The benefit of multigeneration systems comes from the fact that the products can be tailored to whatever the region's customers need locally. If there is a large hospital nearby, a multigeneration plant can be built to provide electricity, space heating, domestic hot water, and oxygen.
Cogeneration and trigeneration plants distribute their product electricity to the grid via wires for use by many customers. The heating and cooling products are generally distributed to buildings in the form of steam, hot water, or chilled water through insulated underground pipes. The system for distributing heating and cooling from a central plant to multiple customers through pipes is called a district energy system or a community energy system, and district heating if hot water or steam is the only distributed product. District energy systems offer a variety of environmental and economic benefits, including:
reduced local air pollution and CO2 emissions
better emission control in a centralized plant
reduced environmental thermal pollution
reduced capital and maintenance costs, since individual heating and cooling equipment isn't needed in homes and buildings
increased overall system efficiencies from a large, centralized plant
increased opportunities to use ozone-friendly cooling technologies (no CFCs)
increased opportunities for electric peak demand reduction through chilled water storage
increase scalability without equipment upgrades
increased fuel flexibility
improved energy security
relocation of air pollution away from municipal areas by eliminating onsite building heating system exhaust
The Intergovernmental Panel on Climate Change (IPCC) identifies cogeneration and district energy as key measures for greenhouse gas reduction.
If the only benefit of a cogeneration plant were to double the efficiency of a natural gas-fired electricity generation plant from about 40% to 80%, that would be a tremendous boon for both the cost of energy to consumers and to the environment. But cogeneration works even without burning fossil hydrocarbons since all that's required is a heat source of sufficient size and temperature. A cogeneration plant can be powered by nuclear, geothermal, biomass, waste incineration, or concentrated solar energy, making it feasible to produce electricity, heating, and cooling without carbon emissions or air pollution (in the case of nuclear, geothermal, and concentrated solar).
Cogeneration/CHP is something I've been interested in since I was 10 years old. It seems like a no-brainer! The benefits of cogeneration are numerous, so why do we even have power plants that don't use cogeneration or trigeneration? One reason is that when many power plants were built, fossil hydrocarbons were cheap and environmental concerns were not as obvious. Cogeneration and district energy systems also require prior planning in order to locate the central plant closer to end users and to engineer the network of pipes to connect buildings. The need for an integrated pipe network makes retrofitting existing municipalities difficult, and the cost may not be competitive with adding different energy sources, such as electricity from solar and wind.
District energy systems and cogeneration are utilized mostly in European countries such as Germany, Denmark, Finland, and others, where up to 35% of total electricity production comes from CHP systems. Unfortunately, the United States has a relatively low adoption of these technologies.
The benefits of cogeneration/trigeneration and district energy systems are numerous, significant, and well-established. They offer a critical puzzle piece to the world's transition to sustainable energy systems. While they are difficult to retrofit into existing systems, new communities can benefit greatly from their use. Not only can they lower the cost of electricity, heating, and cooling for consumers, but they also offer resilience and lower individual capital and maintenance costs by not having to install central boilers or chillers in every building. And, of course, we all benefit from the reduction in air pollution and the reduction in environmental impacts from thermal pollution and greenhouse gas emissions.