Heat Pumps

A heat pump is an environmental energy technology that extracts heat from low temperature sources, upgrades it to a higher temperature and releases it where it is required for space and water heating. Heat pumps can also be operated in a reverse mode for cooling purposes.

There are two common types of heat pumps: air-source heat pumps and Geothermal Heat Pumps (GHPs). Either one can keep your home warm in the winter and cool in the summer. An air-source heat pump pulls its heat indoors from the outdoor air in the winter and from the indoor air in the summer.

An air-source heat pump can provide efficient heating and cooling for your home, especially if you live in a warm climate. When properly installed, an air-source heat pump can deliver two-and-a-half to five times more heat energy to a home compared to the electrical energy it consumes. This is possible because a heat pump moves heat rather than converting it from a fuel, like in combustion heating systems.

The heat pumping cycle can be divided in three steps:

Step 1. A fluid with a boiling point lower than the heat source temperature serves as a medium for heat transport. It is called the working fluid or refrigerant. As the working fluid extracts the heat from the source through a heat exchanger, its temperature rises and it evaporates.

Step 2. Then a compressor compresses the evaporated fluid. Consequently, the pressure and the temperature of the vapour increase. When pumping up a bicycle tyre, you can also observe this phenomenon. The lower side of the pump – where the pressure is highest – is getting very hot.

Step 3. Finally, heat is being transferred from the evaporated fluid to the heat distribution fluid (water or air) in the condenser. As it releases its heat, the working fluid temperature decreases to such a degree that it condenses. After passing through the expansion valve, the fluid regains its initial liquid, low-temperature and low-pressure state. It then flows back to the evaporator where the process starts all over again.

Applications 

In the residential sector

A heat pump does not look very different and can perform the same functions as a conventional gas or oil boiler i.e., space heating and sanitary hot water production.

But it does it much more efficiently, using most of its heating energy from free renewable sources. Renewable heat pumps for space heating are best suited in new houses where high levels of insulation and low temperature heating systems result in low heating demand. In retrofit situations, the heat pump should be installed in parallel with the existing heating system to provide a large proportion of the annual heating needs at reduced operating temperatures.

Water heating is often provided in addition to space heating. The heat pump can provide indirect heating in the domestic hot water cylinder via an internal or external heat exchanger. Because of the high output temperature required, (minimum 55°C) the efficiency for water heating may be reduced. A desuperheater can also be installed between the compressor and the reversing valve of a space conditioning heat pump. It is a refrigerant hot gas-to-water heat exchanger, which is sized to remove the superheat from the compressor discharge gas prior to entry into the refrigerant condenser.

In the commercial sector 

A heat pump is really a three-in-one HVAC system. It combines heating, cooling and air-conditioning in an economical and eco-friendly machine. They are particularly suited for buildings with a high demand for space heating and sanitary hot water production, extensive work-in times and a simultaneous need for cooling.

In large buildings, several individual heat pumps can be placed in different zones and each can be sized to meet the needs of the space it conditions. Some zones of the building may need heating at the same time as other zones need cooling. When properly integrated, a heat pump system can recover excess heat in one zone (sunny side, computer rooms, etc.) and transfer it via a water pipe loop to areas of the building requiring heating. It is, therefore, possible to achieve a balance between heating and cooling needs during a good part of the year.

How is their performance measured?

Energy is needed to activate the heat pump cycle and to compress the vapour for the production of useful heat. The efficiency of this process is expressed by the ratio between the useful heat delivered by the condenser and the driving energy used by the compressor. This ratio is called the Coefficient of Performance (COP).

As environmental heat is free and available in very large quantities, it is not included in the COP. That is why the COP is bigger than 1. The COP of the current generation of heat pumps varies from 2.5 to 5. Since the COP shows performance at a steady state only, a second parameter is usually used to show the performance of the heat pump over an entire year. It is called the Seasonal Performance Factor (SPF), which is the ratio of annually delivered useful heat over annually used driving energy. When calculating the SPF, it is common to include the annual electricity requirements of auxiliary equipment, such as circulation pumps, fans, etc.

The performance of a heat pump system is affected by several factors, which include:

• The climate (annual heating and cooling demand and peak loads)
• The temperature of the heat source and the heating distribution system
• The auxiliary energy consumption
• The heat pump control.

Heat sources

The choice of the heat source is of vital importance for the heat pump, as it will directly influence its application, efficiency (COP & SPF) and initial and operating costs. Heat sources that can be used by heat pumps are air, water and ground.

The main factors that will affect this choice are:

• Its availability: quantity, location relative to need and coincidence with need;
• Its cost: installation, operation and maintenance;
• Its temperature: level (the higher the better) and variation.

Ambient air, the most common heat source for heat pumps, is free and widely available. However, air-source heat pumps achieve on an average 10 to 30% lower seasonal SPF than ground-source or water-source heat pumps. This is mainly due to the rapid fall in capacity and performance with decreasing outdoor temperatures.

A ground-source heat pump uses the earth or ground water or both as sources of renewable heat. The temperature of the ground doesn’t vary very much over the year. This ensures a relatively stable supply of heat for the heat pump and higher performances than air-source ones. Heat is removed from the ground through a collector and transferred to the heat pump via a liquid (water or antifreeze solution).

Open water can also be used as a low temperature heat source. Rivers, streams and lakes, when available, are ideal sources of energy. They have the advantage of needing much less collector surface area than for a ground-source heat pump.

The table above presents the advantages and disadvantages of the most common heat sources:

Refrigerants 

The working fluid in a heat pump must be chosen with consideration of a number of different aspects. Some of the working fluids that have been used extensively in heat pumps have been discovered to have severe impact on the environment and have therefore, been subject to international phase out schemes and strict regulation. The refrigerant must fulfil a number of requirements, of which the most essentials are reviewed below.

Chemical stability

The refrigerant has to be completely stabile within the system and ideally quickly decompose to harmless substances in the atmosphere.

Environmental impact, health and safety

Environmental impact due to direct emissions (leakage) must be kept at minimum level. The use of flammable and toxic refrigerants is limited due to regulation and reluctance from the industry.

Thermodynamic properties

• Freezing temperature: well below normal operating conditions
• Critical point and boiling point temperatures have to be appropriate for the application
• Reasonable operating pressures are preferred in order to keep costs at a minimum. High volumetric refrigeration capacity is beneficial.

Practical characteristics

• High oil solubility is in general preferred
• Compatibility with common construction material
• Low cost.

Maintaining and servicing 

Heat-pump performance will deteriorate without regular maintenance and service. The difference between the energy consumption of a well-maintained heat pump and a severely neglected one ranges from 10 to 25%.

Regular maintenance

Either the homeowner or service technician can perform the following routine maintenance tasks:

• Clean or replace filters regularly (every 2 to 6 months, depending on operating time and amount of dust in the environment).
• Clean outdoor coils as often as necessary (when dirt is visible on the outside of the coil).
• Remove plant life and debris from around the outdoor unit.
• Clean evaporator coil and condensate pan every 2 to 4 years.
• Clean the blower’s fan blades
• Clean supply and return registers and straighten their fins.