Micro combined heat and power

From HealthKB

Micro combined heat and power or microCHP is an extension of the now well established idea of cogeneration to the single/multi family home or small office building.

Overview

In the majority of energy applications, energy is required in multiple forms. These energy forms typically include some combination of: heating, ventilation, and air conditioning, mechanical energy and electric power. Often, these additional forms of energy are produced by a heat engine, running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to the second law of thermodynamics, therefore a heat engine will always produce a surplus of low-temperature heat. This is commonly referred to as "waste heat" or "secondary heat", or "low-grade heat". This heat is useful for the majority of heating applications, however, it is usually not considered practical to transport heat energy over long distances, unlike electricity or fuel energy. By transporting fuel, however, the "waste heat" is essentially being transported along with the fuel, before the waste heat is actually produced.

To make efficient use of energy, the "waste heat" must be used purposefully. Since it is practical to transport electricity, but impractical to transport waste heat, an energy efficient system must generate electricity only at locations where the waste heat can be put to good use. In a central power plant, the supply of "waste heat" often exceeds the demand, so the waste heat has very little economic value. Then the waste heat is typically dissipated in a cooling tower without ever being used purposefully. One way to make better use of the "waste heat", is to consume the primary energy source on-site, and thus generate energy in all of the required forms, at the point-of-use. This is known as a combined heat and power (CHP) system, or "cogeneration".

CHP systems are able to increase the total energy utilization of primary energy sources, such as fuel and concentrated solar thermal energy. Thus CHP has been steadily gaining popularity in all sectors of the energy economy, due to the increased costs of fuels, particularly oil-based fuels, and due to environmental concerns, particularly climate change.

In a traditional power plant delivering electricity to consumers, about 30% of the heat content of the primary heat energy source, such as biomass, coal, solar thermal, natural gas, petroleum or uranium, reaches the consumer, although the efficiency can be 20% for very old plants and 50% for newer plants. In contrast, a CHP system typically converts 10%-20% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. Typically about 10%-30% of the heat is dissipated without being used. In total, at least 65% and often as much as 90% of the heat from the primary energy source goes to useful purposes.

CHP systems have benefited the industrial sector since the energy crisis of the 1970s. For three decades, these larger CHP systems were more economically justifiable than micro-CHP, due to the economy of scale. After the year 2000, micro-CHP has become cost effective in many markets around the world, due to rising energy costs. The development of micro-CHP systems has also been facilitated by recent technological developments of small heat engines. This includes improved performance and/or cost-effectiveness of Stirling engines, steam engines, gas turbines, diesel engines and Otto engines.