What are heat pumps and understand the working?

Gone are the days when people used to lit fireplaces to get cozy in their homes on a cold winter night. With the advancements in technologies, we have seen the transition over the years, from burning wood in fireplaces to using a heater to escape the harsh cold weather of the winter. Not only this, during the summers we have seen the transition from handheld fans to general electric fans to air conditioners, nowadays. The evolution is significant as the demand for necessities increases.

As of 2018, it has been reported that only 1% of US homes are heated via heat pumps. The rest still use fossil fuels like natural gas. The governments are now giving incentives to their population to install a heat pumps.

Heat pumps work on electricity. The efficiency of heat pumps depends on the source of its electricity. For example, solar/hydro energy along with the specific performance of the individual pumps. The older the technology, the more CO2 emissions. Adopting heat pumps to conventional heating would reduce CO2 emissions drastically.

TYPES OF HEAT PUMPS

• AIR-TO-AIR HEAT PUMP
• AIR TO WATER HEAT PUMPS
• GROUND SOURCE HEAT PUMP
• WATER SOURCE HEAT PUMPS
• HYBRID HEAT PUMPS

AIR-TO-AIR HEAT PUMP

These are the most common heat pump types. It is similar to a split AC, with two pieces of equipment one inside and the other outside. These can work as a heating-only device or, alternatively, what better product which can heat as well as cool the surroundings for its user? This is carried out by a reversing valve.

COMPONENTS OF AIR TO AIR HEAT PUMP HEAT PUMP

The main components we have in this type of system are:

• the compressor,
• the reversing valve,
• the indoor heat exchanger,
• an expansion valve with a non-return valve bypass,
• a bi-directional filter drier,
• a sight glass,
• another expansion valve with a non-return valve and bypass,
• the outdoor heat exchanger, a controller, and
• a number of temperature and pressure sensors around the system

WORKING OF AIR TO AIR HEAT PUMP

When heating is to be carried out, the refrigerant leaves the compressor as a high-pressure, high-temperature vapor and passes to the reversing valve. The reversing valve is positioned in heating mode, so the refrigerant passes through this and heads to the indoor unit. Cool air is then blown over the indoor unit's heat exchanger to remove some of the thermal energy and provide heating to the room. As heat is removed, the refrigerant will condense into a liquid.

As energy transfer takes place, the refrigerant which is now liquid and much cooler will leave at high pressure. The refrigerant then comes to the expansion valve and is bypassed. In this mode the expansion valve is closed, so the liquid refrigerant passes through the non-return valve post which passes through the filter drier and sight glass, and then to the second expansion valve. After this, it passes through the expansion valve. As the refrigerant passes through the expansion valve, there is a sudden drop in pressure and the refrigerant expands in volume and turns into a part-liquid, part-vapor mixture. This expansion in volume reduces the temperature and pressure.

The refrigerant then heads to the outdoor heat exchanger. Here, a fan is blowing outside ambient air over the coil and adding heat to the cold refrigerant. The refrigerant boils at a very low temperature(-23.5deg C R134a), and as it boils it will carry away the thermal energy. The refrigerant picks up the thermal energy from the outside air and leaves the outdoor heat exchanger as a low-pressure, low-temperature, slightly superheated vapor, and then heads back to the reversing valve. The reversing valve then diverts this to the compressor to repeat the cycle.

If used for cooling, the system acts like a split air conditioner. The compressor forces the high-pressure, high-temperature vapor refrigerant into the reversing valve. The reversing valve diverts this to the outdoor unit. The fan of the outdoor unit blows ambient air across the heat exchanger. This air will be at a cooler temperature, so it carries the thermal energy of the refrigerant away. The refrigerant condenses as it loses its thermal energy. A high-pressure, slightly cooler liquid is formed after the refrigerant has given up some of its energy. The refrigerant then passes through the non-return valve, which is closed, as the expansion valve is closed. It then passes through the sight glass and the bi-directional filter drier. The next non-return valve is then closed, so the refrigerant passes through the expansion valve. The refrigerant drops in pressure and temperature as it passes through the expansion valve, resulting in a part-liquid, part-vapor mixture. It then flows into the indoor heat exchanger. And in here, a fan blows the warm indoor air over the coil. This causes the heat to transfer from the air into the refrigerant, and so the refrigerant boils and takes its heat away. In a low-pressure, low-temperature, slightly superheated state, refrigerant leaves the indoor unit and flows into the reversing valve. The valve diverts this back to the compressor to repeat the cycle.

WORKING OF AIR TO WATER HEAT PUMPS

These are similar manner to air-to-air heat pumps but without the reversing valve. After leaving the compressor, the high-pressure, high-temperature vapor refrigerant heads into a plate heat exchanger.

On the other side of the plate heat exchanger, water will have been cycled through a hot water storage tank. A heat exchanger absorbs heat from hot refrigerant as cooled water passes through it.

The water will then leave at a much hotter temperature and flow back to the hot water storage tank to repeat this cycle. The refrigerant will condense as it delivers its heat to the water, leaving the heat exchanger as a liquid at a higher pressure and lower temperature. The refrigerant then passes through the filter drier and the sight glass, and then into the expansion valve. The expansion valve causes the refrigerant to become a part-liquid, part-vapor state. It'll be at a low temperature and pressure. It then passes through the outdoor heat exchanger, where the outdoor ambient air causes the refrigerant to boil. After leaving the compressor at low-pressure, low-temperature and slightly superheated vapor, the refrigerant is sucked back into the compressor to repeat the entire process.

The hot water tank then provides hot water to the radiators, sinks, and showers within the building.

WORKING OF  GROUND SOURCE HEAT PUMP

There are two main types of ground source heat pumps, that being the horizontal and the vertical type.

It works the same, the only difference being the access to the heat in the ground varies. The ground source can be used for heating air or water.

The heat pump can also be equipped with a reversing valve and provide either cooling or heating. As in both cases, the outdoor heat exchanger can be a plate exchanger in which refrigerant passes on one side while water and antifreeze cycle on the other. The water and antifreeze mixture is forced by a pump around the pipes within the ground. This will allow it to pick up the thermal energy in heating mode and bring this to the heat exchanger. Having a very low boiling point, the refrigerant on the other side of the heat exchanger absorbs heat, so as it boils, it transports the heat away, so the building can use it.

In the air type system, there can be a reversing valve. The refrigeration system then pulls unwanted heat out of the building and transfer this into the water-antifreeze mixture. As this water is pumped around the pipes in the ground, the heat will be transferred to the ground, where it will return cooler and ready to absorb more heat.

WORKING OF WATER SOURCE HEAT PUMPS

Water source heat pumps come in two main variations, closed and open loops.

In the closed loop, water and antifreeze are mixed and sent to a pond or river in order to collect thermal energy. The same water is then sent round again to repeat the cycle.

The open loop pulls in fresh water from an aquifer or from a river and pumps this into the heat exchanger to collect the heat. Once it passes through, it is then released back into the same water source.

Water and antifreeze are mixed and circulated around pipes to capture thermal energy, which is then transferred to the refrigeration system through the heat exchanger. To cool the building, it will dump the unwanted heat into the water-antifreeze mixture. The unit then works the same as a ground source heat pump.

In an open loop type, the water is pulled in via a pump and sent directly to the heat exchanger. The heat exchanger then pulls the thermal energy out of the water, or it dumps the unwanted heat into the water. The water then passes through the heat exchanger and returns to the source some distance apart.

WORKIN OF HYBRID HEAT PUMPS

According to the outside air temperature, hybrid (or twin source) heat pumps draw heat from different sources.

When outdoor air is above 4 to 8 degrees Celsius (depending on ground water temperature) they use air; at colder temperatures, they use the ground source. While the heat pump is not running, ground source water can be run through the air exchanger or through the building heater exchanger to store summer heat.

This has two advantages: 

It functions as a low-cost system for interior air cooling, and (if groundwater is relatively stagnant) it increases the temperature of the ground source, which improves the energy efficiency of the heat pump system by roughly 4% for each degree in a temperature rise of the ground source.

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