by Jay Stech, PE
A Combine Heat and Power (CHP) unit, also known as Cogeneration, is a fairly straightforward concept. Simply put, it is the joint production of electrical power and the resulting thermal energy from a single source of fuel. Most of us use this concept every day without much thought. A standard automobile uses this exact process. The engine uses gasoline as a fuel that is compressed and ignited in the combustion chamber to create the force (through pistons, crankshaft, transmission, etc.) to move the car. The heat from the combustion of the fuel must be removed from the engine block and is done so by the flow of a coolant (water) through small casings known as ‘water jackets’. The coolant flows back into the radiator where moving air across the fins rejects the excess heat. On a cold January morning or a hot July afternoon, the driver of that car would be pretty uncomfortable if the process stopped there. Instead, the coolant from the radiator is also pumped through additional compressors, evaporators, fans, etc., to blow hot or cold air into the cabin to maintain an appropriate temperature.
On a larger scale, these same concepts are used to produce the electrical, heating and cooling needs of a building or buildings using multiple different fuel sources and engines or turbines. There are significant advantages of using this technology.
With a Conventional Base Load Generation plant, most of the energy used to produce the electrical output is wasted. Of the amount of fuel used (coal, uranium, natural gas, etc.), less than 50% of the resulting energy actually makes it to the end user. Looking at the map of generating stations here in Georgia. There is quite a bit of space between the generating stations, especially in the southern part, and the more than 10 million people it has to get electricity to. This means there are long runs of high voltage transmission lines, large power transformers, smaller medium voltage distribution lines and smaller step-down transformer to get the power to the TV in your house. All those pieces of equipment use energy along the way in the form of heat and other losses.
A Combined Heat and Power (CHP) plant can increase efficiencies to greater than 90%. By utilizing on-site generation to produce the needed electricity, transmission losses are lessened. Instead of generating power and sending it miles away, power can be produced on the premises and depending on the amount of power required can be generated at as low as 480V.
In addition, the heat produced by the combustion of the fuel can be recaptured from the jacket water and exhaust stack. Heat exchangers can use this recaptured heat to provide hot water or steam to be used in facility HVAC systems.
CHP systems also provide resiliency for facilities requiring critical electrical service. CHP systems can be grid connected allowing them to sync to the utility and share the end-user electrical demand. This is a useful feature for peak load shaving to reduce utility rate costs. In addition, CHP systems can be designed to ‘island’ a facility or campus. Islanding is defined as the configuration in which a generator continues to provide power to a user even though the electrical grid is no longer available. Whatever the interruption (i.e. a branch falls on a power line during a storm), CHP systems have the ability to be removed from the grid and provide the needed electrical and heating loads to the user.