Definitely Heat Pumps!

Heat Pumps

A heat pump is a device that transfers heat energy from a low-temperature reservoir to a high temperature reservoir. Its function is basically the reverse of the heat engine. Example is the refrigerator and the air conditioner. 

air conditioner (a) refrigerator (b) 

What is Heat Pump?

A heat pump is a device that uses a small amount of energy to move heat from one location to another. Heat pumps are usually used to pull heat out of the air or ground to heat a home or office building, or they can be switched into reverse to cool a building.
 If you know how an air conditioner works, you already know a lot about how a heat pump works, because heat pumps and air conditioners operate in very similar ways.

Heat pumps are a unique kind of heating system, because they can do the work of both a furnace and an air conditioner. Thus, there's no need to install separate systems to heat and cool your home. Heat pumps can also work extremely efficiently, because they simply transfer heat, rather than burn fuel to create it.

The Heat Pump Refrigeration Cycle

A refrigerant is a fluid, which vaporizes (boils) at a low temperature. The refrigerant circulates through tubes (refrigerant lines) that travel throughout the heat pump. We'll begin our description of the refrigeration cycle at point A on the illustration below, which describes the heat pump when it is heating the house.


 At point A the refrigerant is a cold liquid, colder than the outdoor air. The refrigerant flows to the outdoor coil (point B). This coil is a "heat exchanger" with a large surface area to absorb heat from the air into the colder refrigerant. The heat added to the refrigerant causes the fluid to vaporize, so this heat exchanger is called the "evaporator coil" during the heating cycle. When materials change state (in this case from liquid to gas), large amounts of energy transfer take place. At point C the refrigerant is a cool gas, having been warmed and vaporized by the outdoor air. It is too cool to warm the house, so that's where the compressor (point D) comes in. The compressor raises the pressure of the gas. When that happens, the gas temperature rises. One way to think about it is that the compressor concentrates the heat energy. The compressor is often thought of as the "heart" of the heat pump, since it does most of the work of forcing heat "uphill." The compressor also forces the now hotgas (point E) further into the cycle. The indoor coil (point F) is where the refrigerant gives up its heat to the indoor air. A fan blows air past the indoor coil to distribute heat to the house. This cools the refrigerant to the point where much of it condenses, forming a liquid. In the heating season, the indoor coil is called the "condenser coil." This change of state results in a large transfer of heat energy. The warm mixture of liquid and gas (point G) continues through the cycle to point H, the expansion device (sometimes called a "metering device"). This device reduces the pressure, causing the refrigerant, to become cold again - cold enough so that it is once again ready to absorb heat from the cool outdoor air and repeat the cycle.

How does a Heat Pump work?

As the term "pump" implies, a heat pump moves heat from one place to another. It reverses the natural flow of heat from a warmer to a cooler place. Heat pumps use the refrigeration cycle to accomplish this. The advantage of pumping heat is that it takes less electrical energy than it does to convert electrical energy into heat (as in electric furnaces, baseboards and radiant heaters). In fact-in mild winter temperatures you can get three times as much heat out of each watt of electricity as you get from an electric furnace. Energy cost savings differ for each installation. Properly sized and installed heat pumps can reduce heating costs by 30 to 50 percent compared to electric furnaces.
Heat pumps are named for their source of heat. Air-source heat pumps get heat from the outdoor air. There also are water-source heat pumps, which get heat from water, usually well water. Ground-source heat pumps get their heat from ground below the frost line. Most heat pumps have two main parts; the outdoor unit and the indoor unit. The outdoor unit includes the outdoor heat exchanger, the compressor and a fan. This is where the heat from the air outside is picked up during the heating season, and where the heat from inside the house is rejected during the cooling season. The indoor unit contains the indoor heat exchanger and the fan that distributes heated or cooled air to the distribution system of the house. Some systems have a second indoor cabinet that contains the compressor.