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- 1 August 2007 -
Heat pump technology for energy efficient buildings
How do you keep everyone in your facility comfortable when one area of the building is always hot and another freezing cold? Andy Mayes of Clivet UK gives a round-up of some of the latest developments in reverse cycle water source heat pumps for providing balanced heating in buildings.
The issue of heating and cooling industrial, commercial and residential properties efficiently at all times of the year can be a considerable challenge for owners and building services managers. Centrifugal pumps have long been the mainstay for moving water around in the HVAC (heating/ ventilating air-conditioning) industry and over the years, manufacturers have made efforts to make their pumps more energy efficient. These efforts have included using computational fluid dynamics (CFD) design to improve flow performance and using energy efficient motors and variable speed drives.
Also to be addressed is the issue of how environmentally friendly is the traditional boiler and pump combination in terms of carbon emissions. In larger buildings and building complexes, pumping hot water around is not always cost efficient, nor is it the best way to achieve optimum temperature control. Without question there are some highly efficient HVAC pumps around, and as the Patterson Pump Company states in its promotional material for its Pro-Max pump: "If a lot of water has to be moved efficiently and with a minimum of fuss, no other pump can outperform the horizontal split case pump."
That may be so, but if the ultimate goal is to have balanced heating and cooling around a building, then there is an economic alternative - the heat pump. In the true sense of word, a heat pump is not actually a pump. A heat pump is a device for transferring energy in the form of useful heat from one place to another. It cannot store, make or destroy heat energy - it simply moves it. However, what cannot be overlooked is the massive global increase in sales of heat pumps and the number of companies enetering the market with new products.
Definition
There are a number of definitions for a heat pump, but possibly the European Standard EN14511 provides the most succinct. This says the heat pump is an: 'encased assembly or assemblies designed as a unit to provide delivery of heat. It includes an electrically operated refrigeration system for heating. It can have means for cooling, circulating, cleaning and dehumidifying the air. The cooling is by means of reversing the refrigeration cycle.'
The heat pump has been around for more than 50 years and is a well-proven technology capable of providing safe, reliable heating and cooling at affordable prices. When used for heating building, heat pumps are capable of highly cost-efficient energy applications because they tap into a limitless supply of clean, pollution-free heat, either the surrounding air or heat captured in the ground. The only running costs are those associated with the energy used in transporting the heat, and in some applications most of this energy can also be reclaimed.
There are five main categories of heat pump, these being: air to air, air to water, water to air, ground to air and ground to water. Air to air uses air as the heat source and air is also how the heat is delivered. This type is widely used in commercial buildings as a reverse cycle heat pump that delivers both heating and cooling. Air to water is also a sound commercial proposition and is normally used in conjunction with a fan coil unit.
Water to air can use boreholes, but can also be configured as many units connected together on a common closed water loop to enable energy transfer from hot to cold points in a building. Ground to air uses the stable ground temperature as the heat source with warm air being delivered to the space. Ground to water is a similar system, but utilises under floor heating systems, medium temperature radiators or fan coil units.
How does a heat pump work?
Heat pumps are effective solutions to heating and cooling applications for all types of buildings, domestic, commercial and retail premises including hotels and residential complexes. Significantly, they are a low carbon technology.
The purpose of a heat pump is to absorb heat from an abundant source, then to transport and release it in another location where it can be used for space or water heating. Useful heat can be found in the air outdoors, in the ground and is present in water, rivers, lakes and the sea. Even on the coldest days, sufficient heat is present to warm commercial and domestic premises and what's more, it is free.
The only financial investment is in purchasing and installing the machine to recover it, together with the cost of the energy to run the machine. Even then the savings continue. Modern heat pumps allow a significant quantity of the electrical energy that drives the heat pump to be returned to the building as useful heat.
At the heart of a modern heat pump is a refrigeration system. Paradoxically, the refrigeration cycle is an efficient provider of heat as well as cooling and the basics of its operation are quite easily understood. There are two principle locations in the transfer of heat; the place where heat is absorbed, (the source), and where it is rejected, (the destination). The compressor in the refrigeration system also produces waste heat and a significant proportion of this can be recovered, thereby reducing running costs and the ultimate release of CO 2 .
The mechanical refrigeration cycle consists of an arrangement of heat exchangers; one that absorbs heat, the other that rejects it. All but the largest industrial systems are hermetically sealed and pressurised, thereby reducing noise, space and heat losses.
The heat absorbed is transported through a sealed system of refrigerant pipes the refrigerant being circulated by a compressor. A metering device to control the flow of refrigerant completes the arrangement and it is all connected by pipes. As the refrigerant works under pressure, the whole system is sealed for life.
In order to absorb and release the heat into and from the refrigerant, the ability of the refrigerant fluid to boil from a liquid to a vapour and then to condense back into a liquid is exploited. This is a continual process while the compressor is running and circulating the refrigerant. For all volatile substances, there is a known relationship between its pressure and its boiling point; by controlling these in the refrigerant it is possible to achieve cooling and heating in the same machine at the same time.
High pressure liquid refrigerant is fed through the metering device into the evaporator heat exchanger where it evaporates into a vapour by absorption of heat from the heat source (air, water, ground, other) passing through the heat exchanger. The relatively cool return vapour is drawn back to the compressor, which together with the electric motor that drives it, is contained in a fully sealed hermetic shell. The cooled return vapour from the evaporator passes over the compressor motor windings within the heat pump, thus cooling the windings of the motor. Much of the energy absorbed by the electric motor driving the compressor is absorbed into the refrigerant.
The combined heat from the source, plus much of the waste energy from the electric motor is then compressed to a high temperature vapour and enters the condenser heat exchanger where it is cooled and condensed into a high pressure liquid ready to begin the cycle again. The heat released during the process of condensing the refrigerant to a liquid is rejected via the heat exchanger directly into air or transferred to water to heat the building. The air or water temperature at this point could be 43°C to 60°C, depending on the design of the system.
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