1-تقرير عن heat pump

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.

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.



HEAT PUMP EFFICIENCY


As we saw earlier, a heat pump may use only one-third as much energy as electric resistance heat (electric furnace and baseboards, for example) during mild winter weather (outdoor temperature about 45 degrees F). In the heat pump industry, this is described as a COP (Coefficient of Performance) of 3. COP is the ratio of heat output, to electrical energy input. A number of factors affect COPs of heat pumps; Air Temperature - Heat pumps operate at temperatures colder than 45 degrees F much of the winter. When the temperature is 20 degrees F the COP of the heat pump will be closer to 2 than 3. Frost - Because there is very cold refrigerant flowing through the outdoor heat exchanger, ice can form on the coils, just as it does in freezers. When outdoor temperatures get below 40 degrees F the heat pump may need to be defrosted periodically. To melt the ice, the heat pump takes heat from the house to heat the outdoor coils. This reduces the average heat pump efficiency. Supplemental heat - As it gets colder outside, the heat pump provides less heat, yet the house needs more heat to keep it comfortable. At some outdoor temperature it will be too cold for the heat pump to provide all the heat the house needs. To make up the difference, heat pumps have a supplemental heating system, usually electric resistance coils (basically an electric furnace inside the heat pump indoor cabinet). This part of the system is sometimes called "back-up" or "emergency" heat because the same coils can be used to provide some or all the heat in the event of heat pump failure. Since the supplemental electric heating system doesn't operate with the same efficiency as the heat pump (the COP of electric resistance heat is 1), the total heat pump COP will be much lower when the supplemental heating is on. Gas and oil furnaces provide supplemental heat in some new homes with heat pumps. Existing gas and oil furnaces can also be used as supplemental heat with "add-on" heat pumps that allow a heat pump to be added to an existing system. Controls for these systems are different since the combustion system and the heat pump don't operate at the same time.