| Heat recovery is key to maximising HVAC efficiency |
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Posted: Tuesday, October 26, 2010 3:48 pm
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The challenge of reducing the carbon footprint produced by today’s commercial buildings is a constant consideration in the property and construction market place. Legislation, fuel bills and the financial incentives for carbon reduction are at the forefront of the industry’s collective consciousness and any strategy to help ease the financial and legislative burden is to be embraced. One such way is through heat recovery and “whole buildings thinking” when designing HVAC systems. Here John Durbin, Engineering Department Manager at Daikin UK, looks at how this approach can deliver outstanding levels of efficiency.
Many people will argue that zero heat rejection design is still more of an environmental aspiration than an operational reality. While there is undoubtedly some truth in this approach, a combination of innovative design and technological advances is moving today’s buildings nearer to this utopia. This is welcome news given the introduction of legislation, such as the CRC Energy Efficiency Scheme and changes to Building Regulations, combined with the rapidly increasing cost of fossil fuels.
Until recently, impressive COPs of three to five produced by heat pump technology were the best a HVAC system was likely to produce. Now, however, these pale into comparison with the efficiency levels that can now be achieved. Indeed at Daikin we can achieve COPs of 10.07*. Whilst these figures are achieved under test conditions, huge strides are still possible in improving real-life efficiency levels and it is possible for these double digit efficiencies to become a commercial reality.
To achieve outstanding figures such as these, however, an intelligent approach to the whole building’s needs must be taken. It is no longer good enough to looking at different areas of a building in isolation. The first step towards this is to incorporate heat recovery.
The technology of reclaiming heat from one area within the building and transferring it to another has come a long way since its introduction in the wake of the energy crisis of the 1970s. Then, it was seen as an add-on or niche technology and not considered a serious part of a building’s climate control systems. But if we are to make serious efficiency gains, we need to incorporate heat recovery as an integral part of a well designed and fully integrated climate control system.
Every day, many hundreds of tonnes of CO2 are released by waste heat being rejected into the air. It is easy to see that if we capture this waste heat and reuse it where it is needed then we will achieve the highest levels of efficiency. We need to embrace a “whole building thinking” approach in which heat recovery becomes a vital part of a virtuous circle of climate control.
By taking unwanted heat generated from items such as IT suites or telecoms equipment and reusing it elsewhere in the building, heat can be generated at a much lower cost as well as drastically reducing carbon emissions. The aim must be to reject heat only after every avenue has been explored where it can be used.
The reclamation of heat can be achieved by certain heat pumps that divert heat from indoor units in cooling mode, to areas that require heating. Air source heat pumps, which are cheaper to install than ground source heat pumps, can be used in both new build and renovation projects to provide a highly energy efficient solution. They can achieve significant energy cost savings, compared to traditional fossil fuel boiler systems, by using solar heated air to produce 65-75% renewable heat for buildings and services.
Their effectiveness can be maximised by the use of multiple zones on one circuit. Flexibility means you can optimise the design of every project, so you can cool IT and server rooms, provide hot water for sinks and radiators, use heat reclamation for ventilation and re-use heat in entrances requiring air curtains.
Variable Refrigerant Volume (VRV) technology has been key to this system of heat recovery for some time. Its great advantage for commercial environments is that it varies the refrigerant volume within the air conditioning system to match precisely the building’s requirements at any moment. This allows each area to continually maintain its set temperature, while saving both money and carbon emissions, because it only uses the minimum level of energy required.
To achieve the very highest levels of efficiency, balanced mode operation is achieved by cooling an area of the building that is experiencing the highest heat gains and transferring that recovered heat to other areas of the building which require heating (or hot water provision). It is important that the design process ensures that the indoor units are arranged to maximise the occasions when this balanced operation can take place, with the heat recovery system diverting recovered heat to wherever it is needed, thus contributing significantly to the goal of zero heat rejection.
However, to achieve this near-perfect equilibrium, it is vital to analyse right from the start a building’s multiple requirements, usage patterns and varying occupancy levels, in order to design a fully integrated system that optimizes energy efficiency and heat recovery. Consultants and engineers need to be involved at the earliest possible stages of the design process in order to help architects and systems designers make the right decisions and incorporate the latest thinking on heat recovery within the building modelling process.
By taking this approach from the outset, project teams and their clients will discover the significant impact that integrated heat recovery solutions can have on the reduction of energy usage and will be able to reproduce efficiency levels that until recently were merely hypothetical aspirations.
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