Most of the energy consumed by buildings is in the form of electricity and heat. Typically 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 increases to 65-85%. This concept is called combined heat and power (CHP) or cogeneration.
Like all other generating and heating systems, CHP installations can be designed for a wide range of loads according to the needs of the building. This article describes the main benefits of CHP, as well as the main system configurations available.
Combined heat and power: when is it viable?
As described previously, CHP systems operate more efficiently than separate generating and heating equipment, reducing the building's energy expenses. However, cogeneration is not cost-effective if it only requires one of the outputs – the greatest efficiency can only be achieved by using both.
A CHP system also makes the installation less dependent on the electrical grid while eliminating transmission and distribution losses. You can also expect more predictable electricity expenses, as the impact of changes in kWh prices is lessened or completely eliminated.
Take an energy audit to find out if CHP is viable for your building.
When large energy consumers implement cogeneration, there is also a benefit for energy network operators: the network is decongested. This helps the utility provide better service to other customers who do not generate their own electricity, while delaying costly upgrades to transmission and distribution infrastructure.
CHP systems can also be combined with an absorption chiller, a special type of chiller that works with heat input instead of an electrical compressor. In this case you have a trigeneration system, which adds refrigeration to the CHP. Absorption chillers have applications in space cooling, industrial process cooling and refrigeration.
CHP System Settings
CHP is a general concept and there are many viable system configurations. They all have the common goal of providing heat and electrical energy, but the equipment used varies significantly. Most CHP systems use one of the following types of equipment:
- Reciprocating engines
- gas turbines
- steam turbines
- Microturbines
- fuel cells
According to the US Department of Energy, reciprocating engines are the most common CHP system configuration, found in more than 50% of designs. However, gas turbines gain in installed capacity, accounting for more than 60%.
Reciprocating engines
Typical capacity range: 10 kW to 10 MW
Electrical-only efficiency: 30-42%
Cogeneration efficiency: 77-83%
Reciprocating engines are based on the same principle as car engines, but deployed on 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, cooling water, or lubricating oil. The motors offer flexible operation as they can operate at partial load without a significant drop in efficiency.
As a technology, alternative engines are very mature and their supply chain is well established. Globally, more than 200 million units are manufactured and deployed each year.
Gas Turbines
Typical capacity range: 1 to 300 MW
Electrical-only efficiency: 24-36%
Cogeneration efficiency: 65-71%
Gas turbines become viable for cogeneration applications when the design is large enough to justify a few megawatts of capacity. They are particularly useful when industrial processes require large amounts of heat, as the high-temperature exhaust from the turbine can be used directly. Ideally, there should be a constant demand for heat and electricity, as the efficiency of a gas turbine decreases drastically under partial load conditions.
Although gas turbines are normally associated with generating electricity, they are also used to propel vehicles and drive equipment such as compressors and pumps.
steam turbines
Typical capacity range: 100 kW to 250 MW
Electrical-only efficiency: 5-7%
Cogeneration efficiency: 80%
Steam turbines are best suited for cogeneration applications where the heating load is significantly greater than the electrical load. Unlike reciprocating engines and gas turbines, steam turbines are not directly exposed to fuel combustion – it occurs separately in a boiler.
CHP systems with steam turbines are commonly used when a source of cheap fuel such as wood chips and other forms of biomass exists. These turbines do not suffer drastic loss of efficiency at part load, offering flexible operation.
Microturbines
Typical capacity range: 30 to 330 kW
Electrical-only efficiency: 25-29%
Cogeneration efficiency: 64-72%
Microturbines are intended for smaller-scale applications than conventional gas and steam turbines. They feature a modular design 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, as their partial load efficiency is poor.
Fuel Cells
Typical capacity range: 5 kW to 2.8 MW
Electrical-only efficiency: 38-42%
Cogeneration efficiency: 62-75%
Fuel cells have a fundamental difference from the previously mentioned CHP technologies: there is no combustion and the fuel undergoes a direct chemical reaction to produce heat and electricity. As a result, fuel cell exhaust 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 small loss of efficiency under partial load. They are also quieter than other CHP options since they do not have rotating machines.
Final Observations
Large properties with a constant demand for electricity and heat can reduce their energy expenses through the use of combined heat and power (CHP). However, the first step should be an energy audit, which provides a clear picture of the building's energy consumption. Based on this data, it is possible to determine whether CHP is viable, selecting the most appropriate system configuration, if applicable.
