Most of the energy consumed by buildings is in the form of electricity and heat. Normally, these inputs are produced independently, which results in an efficiency of 45-55% according to the US Department of Energy. However, when heat and electricity are produced from the same input, efficiency is improved to the range of 65-85%. This concept is called combined heat and power (CHP), or cogeneration.
Like all other generation and heating systems, CHP installations can be designed for a wide range of loads according to building needs. This article describes the main benefits from CHP, as well as the main system configurations available.
As previously described, CHP systems operate at a higher efficiency than separate generation and heating equipment, reducing building energy expenses. However, CHP is not cost-effective if you only require one of the outputs - the higher efficiency can only be achieved if you use both.
A CHP system also makes the facility less reliant on the power grid, while eliminating transmission and distribution losses. You can also expect more predictable electricity expenses, since the impact of kWh price changes is diminished or eliminated completely.
When large energy consumers deploy CHP, there is also a benefit for power network operators: the grid is de-congested. This helps the utility company deliver a better service to other clients who do not generate their own electricity, while deferring expensive upgrades to the transmission and distribution infrastructure.
CHP systems can also be combined with an absorption chiller, a special type of chiller that works with a heat input instead of an electric compressor. In this case you have a trigeneration system, which adds cooling to CHP. Absorption chillers have applications in space cooling, industrial process cooling and refrigeration.
CHP is a general concept, and there are many viable system configurations. All have the common purpose of delivering heat and electric power, but the equipment used varies significantly. Most CHP systems use one of the following types of equipment:
According to the US Department of Energy, reciprocating engines are the most common CHP system configuration, found in over 50% of projects. However, gas turbines win in terms of installed capacity, accounting for more than 60%.
Typical capacity range: 10 kW to 10 MW
Electric-only efficiency: 30-42%
CHP efficiency: 77-83%
Reciprocating engines are based on the same principle as car engines, but deployed at a larger scale. Shaft rotation is achieved with a series of pistons that follow a four-stroke movement: intake, compression, power and exhaust.
When a reciprocating engine generates electricity, heat can be recovered from three sources: directly from the engine exhaust, from cooling water, or from the lubricating oil. The engines offer flexible operation, since they can run at part load without a significant drop in efficiency.
As a technology, reciprocating engines are very mature and their supply chain is well established. Globally, more than 200 million units are manufactured and deployed each year.
Typical capacity range: 1 to 300 MW
Electric-only efficiency: 24-36%
CHP efficiency: 65-71%
Gas turbines become viable for CHP applications when the project is large enough to justify a few megawatts of capacity. They are particularly useful when industrial processes require large amounts of heat, since the high-temperature turbine exhaust can be used directly. Ideally, there should be a constant demand for heat and electricity, since the efficiency of a gas turbine decreases dramatically under part-load conditions.
Although gas turbines are normally associated with electricity generation, they are also used in vehicle propulsion and to drive equipment such as compressors and pumps.
Typical capacity range: 100 kW to 250 MW
Electric-only efficiency: 5-7%
CHP efficiency: 80%
Steam turbines are better suited for CHP applications where the heating load is significantly larger than the electric load. Unlike reciprocating engines and gas turbines, steam turbines are not exposed directly to fuel combustion - it occurs separately in a boiler.
CHP systems with steam turbines are commonly used when there is a source of cheap fuel, such as wood chips and other forms of biomass. These turbines do not suffer a drastic efficiency loss at part load, offering flexible operation.
Typical capacity range: 30 to 330 kW
Electric-only efficiency: 25-29%
CHP efficiency: 64-72%
Microturbines are intended for smaller-scale applications than conventional gas and steam turbines. The come with a modular design that is ideal for buildings with planned expansions, distributed energy systems and microgrids. Like conventional gas turbines, microturbines are intended for applications where their full output can be used continuously, since their part-load efficiency is poor.
Typical capacity range: 5 kW to 2.8 MW
Electric-only efficiency: 38-42%
CHP efficiency: 62-75%
Fuel cells have a key difference with the CHP technologies mentioned previously: there is no combustion, and the fuel undergoes a direct chemical reaction to produce heat and electricity. As a result, the exhaust of fuel cells is mostly carbon dioxide, free from more harmful compounds like nitrogen oxides, volatile organic compounds and the deadly carbon monoxide.
Fuel cells offer flexible operation, experiencing only a minor efficiency loss under part load. They are also more silent than other CHP options, since there is no rotating machinery.
Large properties with a constant demand for electricity and heat can reduce their energy expenses by deploying combined heat and power (CHP). However, the first step should be an energy audit, which provides a clear snapshot of building energy consumption. Based on this data, it is possible to determine if CHP is viable, selecting the most suitable system configuration if that is the case.
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