Combined Heat and Power Systems (CHP) for Buildings

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Combined Heat and Power (CHP) is a technology that produces electricity and thermal energy at high efficiencies using a heat engine or power system. CHP, also referred to as cogeneration, is a process in which the heat produced as a by-product of power generation is reused efficiently.

CHP is used for individual facilities, buildings, microgrids, utility resources, etc. It is an energy-efficient element incorporated in the design of net-zero energy buildings.

In the U.S, a total of 4300 CHP systems are installed across all the 50 states. It is recognized as a promising technology to reduce carbon emissions and a reliable source of power generation during emergencies.

This article discusses the features and applications of CHP technology in building construction.

Features of CHP Technology for Building Facility

• CHP converts the otherwise-wasted thermal energy into useful mechanical and thermal energy in a single integrated system. It helps to increase energy efficiency and decrease unnecessary fuel consumption.
• CHP is appropriate for continuous local demand for heat and power. It is installed in district heating schemes, industrial applications, hotels, and leisure centers or applications that demand heat for manufacturing processes. If excess electricity is generated, it is exported back to the national grid.
• The individual parts of a CHP system are:
  • Prime mover
  • Generator
  • Heat recovery unit
  • Electrical Interconnection
• All these parts are configured into an integrated whole. The prime mover of a CHP system is the engine, turbine, or fuel cell that drives the whole system. 
• Figure-2 shows a combustion turbine or reciprocating engine CHP system. The system burns fuel to produce electricity. The heat emitted from the turbine or engine is captured by the heat recovery unit and converted to thermal energy. This is used in the form of steam or hot water. 

• The prime movers used in the CHP systems are reciprocating engines, gas turbines, boiler/steam turbines, microturbines, fuel cells, etc. 
• CHP plant is manufactured as packaged units or custom-designed and built. 
• CHP can be configured as a topping or bottoming cycle. 

Topping Cycle CHP System

In this CHP system, the fuel is combusted in a primer mover like a reciprocating engine, combustion or gas turbines, fuel cells, etc., that drives the whole system to generate electricity. 

Bottoming Cycle CHP System

This is also called “waste heat to power (WHP),” where the fuel is combusted to provide thermal input to a furnace or other industrial process. The heat that is not used for the process is used to produce electricity.

• The total cost of installation of CHP = Total cost of equipment + Cost of Installation, labor, and materials + Cost of engineering and project management + Carrying costs during construction.
• The non-fuel operation and maintenance (O&M) costs of CHP systems include routine inspections, scheduled overhauls, operating labor, and preventive maintenance.
• The start-up times (time taken to get the units online) of CHP units are dependent on the technology and fuel used in the CHP system.
• CHP as a process is not dependent on a specific fuel. Hence, this system works with renewable fuels like biomass.

Advantages of CHP System

1. CHP is sustainable as it reduces carbon emission and air pollutants like sulfur dioxide, volatile organic particles, nitrogen oxide, etc. It produces one-half to one-third of the emissions compared to conventional power plants to generate heat and power.
2. CHP systems have high reliability as they are not vulnerable to disruption to the transmission system and provide power at the time of emergencies during an outage.
3. As per the Department of Energy and Climatic Changes (DECC), CHP has an efficiency of over 80 percent compared to 38 percent for a coal-fired power station.
4. CHP provides 30 percent savings on energy bills.
5. They reduce transmission and distribution losses.

Applications of a CHP system

1. Commercial buildings like hotels, nursing homes, office buildings, retail, etc.
2. Institutional buildings like hospitals, prisons, military bases, educational institutions, etc.
3. Municipal facilities like district energy systems, wastewater treatment facilities, etc.
4. Industrial facilities like chemical, ethanol, agriculture, and food processing units.

The site-specific factors determine whether CHP is a feasible technical and economic solution for a facility.  Even though CHP demands significant up-front investment and careful consideration, it can pay for itself within a few years.

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