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Heat Pump

Heat Pump


You probably own a heat pump without realising it - a refrigerator is actually a type of heat pump. When used for heating, heat pumps save energy by extracting heat from an outside source, and delivering it for use within the building. They can be used for any normal heating need. This apparently magical technology is not new: since 1930 people have been using new several types of  heat pumps in a bid to save energy and fuel costs.  


HOW DOES A HEAT PUMP WORK?


There are many different kinds of heat pumps, but they all operate on the same basic principle of  heat transfer. The most familiar form of heat pump is the domestic refrigerator. Here, heat is extracted from the cabinet to keep food fresh and the extracted heat is expelled through the radiator grill at the back of the unit. In this case the heat is merely a waste product. In the heat pump, we utilise this heat, and put the "cold part" outside.

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. Heat pumps can also work extremely efficiently, because they simply transfer heat, rather than burn fuel to create it. Heat pumps work best in moderate climates. If you live in a moderate climate, using a heat pump instead of a furnace and air conditioner may help you save money on your utility bill. Most heat pumps are somewhat limited by the cold, however, so it is important that you learn which kind of heat pump is best for your area before installing one in your home or office building. If you install the wrong kind of heat pump, you may end up paying even more in energy costs than you do already.

Heat transfer means that rather than burning fuel to create heat, a device moves heat from one place to another. Heat naturally flows downhill, which means that it tends to move from a location with a high temperature to a location with a lower temperature. 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 (e.g. the ground or air) into a heat sink (e.g. your home).

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. This system is more commonly known as an air-air heat pump, because it takes heat from outdoor air and transfers it to indoor air ducts. With the proper modification, air-source systems can also work with other types of indoor heating systems.

The reversing valve is a very versatile part of a heat pump. It reverses the flow of the refrigerant, so that the system begins to operate in the opposite direction. Instead of pumping heat inside your home, the heat pump releases it, just like an air conditioner. The refrigerant now absorbs heat on the indoor side of the unit and flows to the outside, where the heat is released and the refrigerant cools and flows back indoors to pick up more heat.

This technology is not appropriate for every situation, and is mostly viable in remote areas where mains gas is not available. It is particularly suited to well insulated houses with underfloor heating.


Types of Heat Pumps

Electric Air-Source Heat Pumps (ASHPs). ASHPs, often used in moderate climates, use the difference between outdoor and indoor air temperatures to cool and heat. They also have a higher Heating and Seasonal Performance Factor (HSPF), which measures the heating efficiency of the heat pump.

Geothermal Heat Pumps (GHPs). GHPs are similar to air source heat pumps, but use the ground instead of outside air to provide heating, cooling, and often water heating. Because they use the earth's natural heat, they are among the most efficient and comfortable heating and cooling technologies currently available. Although initially expensive, you can achieve significant cost savings on energy bills. GHPs are most often installed in new homes and require a duct system.

The diagram below may help you understand how heat pump helps in getting energy from the surrounding for our internal use.

Extracted heat (2.5kW) + Power input (1kW) = The useful heat output (3.5kW) 


Where the heat comes from.

There are various types of heat sources which broadly fall into the following categories:-


Ground source

Water source

Air source 


How is the heat delivered to the building?

Heat pumps usually deliver heat in the form of hot water, as do most conventional central-heating systems. However, to maintain a high energy-efficiency, the emitter system should be designed so that the water temperature is as low as possible. Ideally, a well-designed underfloor heating system should be used. Such systems are energy efficient and very comfortable. Radiators may be the only alternative, but should be significantly larger in area than normal. Several radiators in one room is advantageous.

Ducted hot air is an alternative method of distributing heat into a building, however, this is often not as comfortable as radiant (underfloor) heating and should be installed with caution. It is particularly undesirable in badly insulated buildings.


COOLING.

In summer, buildings can overheat. The main cause is often sunlight (solar gain). One square metre of sunlight through a window can contribute almost 1kW of heat to the room. This is the same as the heat from a small electric room heater. The solar heat often falls on the floor, and therefore heats the room. Air-to-air systems (air-conditioners), are used throughout the world for air cooling. Their energy consumption is significant, so they are to be avoided if possible. It is far more energy-responsible to reduce the solar gain in the first place by shading the sun. There are many ways to minimise heat build-up in houses including limiting the heat sources and good ventilation. Older buildings that have a high thermal mass tend to have less problems. It is perfectly possible to design modern buildings that keep sufficiently cool without the need for air-conditioning.

If air-conditioning is deemed to be necessary, then a water or ground-coupled heat pump system will be the most energy efficient. This type rejects the heat to the ground coil or borehole.

If the liquid in the ground source is pumped directly around the underfloor heating pipes, then a certain amount of ‘free’ cooling can result. This is known as 'Passive' cooling. It will have a limited effect, and will only work with a borehole, or with a very good ground collector with moving ground water . So consider this option with caution. But if coupled with good housekeeping as outlined above, it can help to curb excessive internal temperatures with minimal use of energy.

Basic description of the component parts of a GSHP: 

1 A heat pump packaged unit: Water-Water (or Brine-Water) type. (approx. the size of a small fridge) containing a pair of cold-water (glycol) and a pair of heated-water connections.

2. The heat source which is usually a closed loop of plastic pipe containing a Glycol Antifreeze solution. This pipe is buried in the ground in vertical bore holes or horizontal trenches. The trenches take either straight pipe or coiled pipe, buried about 1.5 to 2m below the surface. A large area is needed for this.

3. The heat distribution system. This is either underfloor heating pipes or conventional radiators of large area connected via normal water pipes.

4. Electrical input and controls. The system will require an electrical input, three-phase being preferred, but single phase is perfectly adequate for most systems. A specialised controller will be incorporated to provide temperature and timing functions of the system.

This type of installation offers many advantages.

a) The water-water (or antifreeze-water) heat pump unit is a sealed and reliable self contained unit.

b) There are no corrosion or degradation issues with buried plastic pipes.

c) The system will continue to provide the same output even during extremely cold spells.

d) The installation is fairly invisible. i.e. no tanks or outside unit to see.

e) No regular maintenance required.

Heat pump REFRIGERANTS (the working fluid)

As discussed in Heat pump technology, closed-cycle compression type heat pumps require a working fluid. Traditionally, the most common working fluids for heat pumps have been:

•    CFC-12 Low- and medium temperature (max. 80°C);

•    CFC-114 High temperature (max. 120°C);

•    R-500 Medium temperature (max. 80°C);

•    R-502 Low-medium temperature (max. 55°C);

•    HCFC-22 Virtually all reversible and low-temperature heat pumps (max. 55°C).

CFCs belong to the group of prohibited refrigerants. Due to their high ozone depletion potential the manufacture of these refrigerants, and their use in new plants, is now banned although they are still permitted in existing plants.

Heat sources

The technical and economic performance of a heat pump is closely related to the characteristics of the heat source. An ideal heat source for heat pumps in buildings has a high and stable temperature during the heating season, is abundantly available, is not corrosive or polluted, has favourable thermophysical properties, and its utilisation requires low investment and operational costs. In most cases, however, the availability of the heat source is the key factor determining its use. The table on the right below presents commonly used heat sources.

•    Ambient and exhaust air, soil and ground water are practical heat sources for small heat pump systems, while sea/lake/river water, rock (geothermal) and waste water are used for large heat pump systems.

•    Ambient air is free and widely available, and it is the most common heat source for heat pumps.

•    In mild and humid climates, frost will accumulate on the evaporator surface in the temperature range 0-6°C, leading to reduced capacity and performance of the heat pump system.

•    Exhaust (ventilation) air is a common heat source for heat pumps in residential and commercial buildings.

•    In open systems the ground water is pumped up, cooled and then reinjected in a separate well or returned to surface water.

•    Ground-source systems are used for residential and commercial applications, and have similar advantages as (ground) water-source systems, i.e. they have relatively high annual temperatures.


Table 1. commonly used heat source


Heat Source
Temperature Range (°C)
 Ambient air  -10 - 15
 Exhaust air  15 - 25
 Ground water   4 - 10
  Lake water
 0 - 10
 River water  0 - 10
 Sea water  3 - 8
 Rock  0 - 5
Ground  0 - 10
 Waste water and effluent
  >10

•    Rock (geothermal heat) can be used in regions with no or negligible occurrence of ground water. Typical bore hole depth ranges from 100 to 200 metres.

•    River and lake water is in principle a very good heat source, but has the major disadvantage of low temperature in winter (close to 0°C). Great care has to be taken in system design to avoid freezing of the evaporator.

•    Sea water is an excellent heat source under certain conditions, and is mainly used for medium-sized and large heat pump installations.

•    Waste water and effluent are characterised by a relatively high and constant temperature throughout the year. 


Huge Worldwide Future potential

If it is further considered that heat pumps can meet space heating, hot water heating, and cooling needs in all types of buildings, as well as many industrial heating requirements, it is clear that heat pumps have a large and worldwide potential.

  Of the global CO2 emissions that amounted to 22 billion tonnes in 1997, heating in building causes 30% and industrial activities cause 35%. The potential CO2 emissions reduction with heat pumps is calculated as follows:

•    6.6 billion tonnes CO2 come from heating buildings (30% of total emissions).

•    1.0 billion tonnes can be saved by residential and commercial heat pumps, assuming that they can provide 30% of the heating for buildings, with an emission reduction of 50%.

•    A minimum of 0.2 billion tonnes can be saved by industrial heat pumps

The total CO2 reduction potential of 1.2 billion tonnes is about 6% of the global emissions! This is one of the largest that a single technology can offer, and this technology is already available in the marketplace. And with higher efficiencies in power plants as well as for the heat pump itself, the future global emissions saving potential is even 16%.

Heat Pumps -Year-round performance for total home comfort

Providing year-round performance for total home comfort, heat pumps are a great solution for your home comfort system. That’s because they work to provide both heating and cooling. Whether it’s the hottest day of the summer, or the coldest day of winter, Trane heat pumps work day in and day out to keep your family in premium comfort.


 Heat Pumps Save Your Money?

The cost to install and run different kinds of heat pumps varies widely. Geothermal or ground-source heat pumps are more expensive to install than air-source heat pumps, because ground-source pumps require you to dig down to a heat source and involve more complex heat transfer systems. Expect to pay as much as $5,000 to $7,500 for a ground-source heat pump system. Air-source heat pumps can be found for much cheaper, averaging around $1,500 to $4,000, because the units tend to be simpler, and installation is much easier.

The cost required to run and repair a heat pump varies with the type of system. It is less expensive to run a ground-source heat pump, because the ground and water have a relatively constant temperature that allows the heat pump to operate efficiently. Ground-source systems also have the advantage of not being exposed to the outdoor weather, which prevents a lot of wear and tear. On the downside, they can be costly to repair if you need to access an underground portion of the system. Air-source systems are easy to access and service, but they may need more regular maintenance because they are exposed to the elements. Also, air-source heat pumps may use more supplemental energy to run, especially in colder climates, and this will cost you more on your utility bill.

Heat pumps may save you anywhere between 30 and 40 percent or more on your utility bill, but neglect will reduce a heat pump's efficiency over time. It's important to factor in the climate where you will be using the heat pump to make sure you select a system that can run efficiently in your area.

Heat pumps can save you a lot of money on utilities if you install the right kind of pump for your area.


Heat pumps in residential and commercial buildings

Functions

Heat pumps for heating and cooling buildings can be divided into four main categories depending on their operational function:

•    Heating-only heat pumps, providing space heating and/or water heating.

•    Heating and cooling heat pumps, providing both space heating and cooling.

The most common type is the reversible air-to-air heat pump, which either operates in heating or cooling mode. Large heat pumps in commercial/institutional buildings use water loops (hydronic) for heat and cold distribution, so they can provide heating and cooling simultaneously.

•    Integrated heat pump systems, providing space heating, cooling, water heating and sometimes exhaust air heat recovery.

Water heating can be by desuperheating only, or by desuperheating and condenser heating. The latter permits water heating when no space heating or cooling is required.

•    Heat pump water heaters, fully dedicated to water heating.

They often use air from the immediate surroundings as heat source, but can also be exhaust-air heat pumps, or desuperheaters on air-to-air and water-to-air heat pumps. Heat pumps can be both monovalent and bivalent, where monovalent heat pumps meet the annual heating and cooling demand alone, while bivalent heat pumps are sized for 20-60% of the maximum heat load and meet around 50-95% of the annual heating demand (in a European residence). The peak load is met by an auxiliary heating system, often a gas or oil boiler. In larger buildings the heat pump may be used in tandem with a cogeneration system (CHP).

In residential applications room heat pumps can be reversible air-to-air heat pumps (ductless packaged or split type units). The heat pump can also be integrated in a forced-air duct system or a hydronic heat distribution system with floor heating or radiators (central system).

In commercial/institutional buildings the heat pump system can be a central installation connected to an air duct or hydronic system, or a multi-zone system where multiple heat pump units are placed in different zones of the building to provide individual space conditioning. Efficient in large buildings is the water-loop heat pump system, which involves a closed water loop with multiple heat pumps linked to the loop to provide heating and cooling, with a cooling tower and auxiliary heat source as backup.

The different heat sources that can be used for heat pumps in residential and commercial buildings are described in the section Heat sources. The next paragraph describes the types of heat and cold distribution systems that can be used in buildings.


Heat and cold distribution systems

Air is the most common distribution medium in the mature heat pump markets of Japan and the United States. The air is either passed directly into a room by the space-conditioning unit, or distributed through a forced-air ducted system. The output temperature of an air distribution system is usually in the range of 30-50°C. 


Environmental

Global Warming Effects

Any use of fossil fuel creates harmful CO2 emissions. Electricity is mostly produced by burning fuels, and the process of generation is inefficient. Electricity is therefore not a good form of energy for heating. However, the energy advantages of heat pumps can, in many cases, more than compensate for this, and makes them score well with respect to CO2 emissions.

Vast amounts of heat energy are used by heating our houses, and it is always advantageous to minimise the heat requirements by normal conservation measures like insulation, this is certainly the best and simplest first-step. Sadly, we cannot all live in super-insulated eco-houses, so some sort of heat energy input is required.

The graph compares the carbon dioxide emissions from common heating systems (gas, oil and electricity) to that of heat pump systems. The COP (efficiency) for a typical Ground Source heat pump with radiators is 3, but if well designed underfloor is used the efficiency can be 4.

Some heat pumps incorporate a normal electric back-up heater to cope with the coldest periods in the winter. This is more the case with air-source systems since the period of highest heat-demand also corresponds with the time when there is minimum heat available in the air. Therefore, on the coldest day the electricity consumption for some heat pumps will increase many fold, putting a strain on the electricity supply grid. It would be better to use boilers, or ideally, wood stoves as a back-up. It should however be notes that the total annual contribution by back-up heaters is surprisingly small.

The COP (coefficient of performance) of a heat pump is the ratio of input to output. 


How heat pumps achieve energy savings and CO2 emissions reduction - an introduction

Heat pumps and energy saving

Heat pumps offer the most energy-efficient way to provide heating and cooling in many applications, as they can use renewable heat sources in our surroundings. Even at temperatures we consider to be cold, air, ground and water contain useful heat that's continuously replenished by the sun. By applying a little more energy, a heat pump can raise the temperature of this heat energy to the level needed. Similarly, heat pumps can also use waste heat sources, such as from industrial processes, cooling equipment or ventilation air extracted from buildings. A typical electrical heat pump will just need 100 kWh of power to turn 200 kWh of freely available environmental or waste heat into 300 kWh of useful heat.