Monday, August 10, 2009

Heat Pumps


There are many different kinds of heat pumps, but they all operate on the same basic principle of heat transfer. Heat transfer means that rather than burning fuel to create heat, a device moves heat from one place to another.As with many technologies that we use in every-day life; the basic principles of how a heat pump works are simple. Heat naturally flows from a location with a high temperature to a location with a lower temperature. A pie taken from a hot oven and set on the counter will cool. If air is blown past it, it will cool faster. A heat pump uses a small amount of energy to switch that process into reverse, pulling heat out of a relatively low-temperature area, and pumping it into a higher temperature area. In a heat pump, this heat is transferred from a heat source (for example, the ground or air) into a heat sink (for example, your home).
As seen above, one of the most common types of heat pumps is the air-source heat pump, which takes heat from the air outside your home and pumps it inside through refrigerant-filled coils. Inside this basic heat pump, you'll find two fans, refrigerator coils, a reversing valve and a compressor. There are usually two physical units, one indoor and one outdoors they contain the compressor and heat exchanger. The direction of heat flow may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. The effectiveness of a heat pump is based on the temperature difference between the source and the sink and which cycle it is in. Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. This is because the energy used to power the compressor is largely converted to useful heat when in heating mode and released into the house as extra heat. During the cooling cycle, the condenser is normally outdoors, and the compressor's dissipated work is rejected rather than put to a useful purpose.
The most common types of heat pumps work by utilizing the physical properties of the refrigerant, a fluid that can reach a sufficiently high temperature when compressed, since the laws of nature prevents heat from flowing from a cold fluid to a hot heat sink. The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized gas is cooled in a heat exchanger (the indoor unit), which is a series of coils where the hot fluid comes into contact with cooler air, until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device like an expansion valve. This device then passes the low pressure, (almost) liquid refrigerant to another heat exchanger, the evaporator where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.
All our surroundings, even a block of ice, has heat. The purpose of a heat pump is to absorb heat in one place where it is plentiful, then to transport and release it in another location where it can be used for heating. Useful heat can be found in the air outdoors, in the ground, and is present in water, rivers, lakes and the sea. Even on the coldest winter days, sufficient heat is present to warm our homes and buildings. All we have to pay for is the machine to operate which uses less energy than fueling a furnace. Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. Heat pumps are most efficient when the temperature differential is moderate; with a temperature differential less than 20-30 degree range the system would run optimally and be most cost effective. Air heat pumps have typically only been used in moderate climates because their efficiency declines the larger the temperature differential between desired temperature and source temperature. However, there are other types of heat pumps that can overcome these problems.

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