Energy efficient ventilation systems

Air handling represents one of the largest uses of energy in air conditioned/ventilated buildings. For all-air systems used in air conditioned offices for example, the fans can account for up to two thirds of the total energy cost. However, through selection of the appropriate system, equipment and components, and attention to detail at the commissioning stage, substantial savings in long-term energy usage can be realised.

The cost of energy has, in real terms, been falling over the past ten years. Electricity is approximately 36% cheaper and gas is costing 18% less. As a result, energy as a proportion of total central London office service costs has been falling and ranges from approximately 10% for non air conditioned space to around 16% for air conditioned space. However, if staff costs are included in the assessment, then energy typically represents 0·25% to 0·75% of a non-industrial business' costs, which suggests that there is limited commercial justification for significant capital investment in energy saving technology.

However, there are a number of drivers which are encouraging developers and occupiers to invest in more energy efficient installations, including government policy and regulation, the growing perception of the value of a green corporate image and the growing conviction of organisations that they need, and need to be seen to be, enforcing sustainability policies.

The main driver behind increasing energy efficiency is the government's commitment to the reduction in carbon emissions and other greenhouse gases through the Kyoto Protocol. Buildings currently contribute 46% of UK carbon emissions, with 19% originating from non-domestic building types. Government environmental policy in improving the performance of non-domestic buildings is concentrated on three key areas:

• The revised Part L2 of the Building Regulations, which requires either the demonstration of the specification of efficient air conditioning and mechanical ventilation systems, or the achievement of carbon emissions standards.
  
• Climate Change Levy – the levy, in force since April 2001, is a tax on the business use of energy, targeted to raise £1.75 billion per annum. The levy is revenue neutral with £1.7 billion being returned to business via reductions in National Insurance, and £50 million being used to fund Enhanced Capital Allowances.
  
• Enhanced Capital Allowances (ECAs) – the ECA scheme extends the benefit of capital allowances, permitting businesses to claim 100% tax relief for qualifying investments in energy efficient equipment in the first year, thus delivering a significant cash flow boost and shortening an investment's payback period.

Allowances can be claimed against eleven categories of energy saving plant and machinery. While the scheme seems very attractive, assets with a life expectancy exceeding 25 years are excluded, and investment in lighting only qualifies in limited circumstances. Furthermore, the complexity of the claims process may deter some users from making a claim.

Energy efficient ventilation systems
The foundation of energy efficient design is a clear understanding of the client's needs and the required environmental conditions. Specification of base environmental criteria in excess of sector norms such as the British Council for Offices specification can lead to overcapacity, resulting in less efficient operation of plant in the long-term. Without a clear agreement of the client's requirements, it is also possible that energy saving design options may not be considered, again resulting in sub-optimal solutions. Once the need for ventilation or air conditioning has been established, the following issues need to be addressed to ensure efficient operation:

• The plant should not be oversized, which is a requirement of the new Part L2 regulations.
  
• Simultaneous heating and cooling should be avoided, except where humidity control is required.
  
• The hours of operation of systems should be optimised, through a good understanding of how the building will be occupied and an adaptable controls strategy.
  
• Putting in place operational strategies, which ensure that the cooling system is shut down in winter, provided there are no cooling loads, and that in summer that any reheat facility is turned off unless required for humidity control.
  
• Ensure maximum use is made of free cooling through the use of recirculated air, where appropriate.
  
• Where appropriate, design systems to allow for the air volume to vary as the load/occupancy changes within the space, such as in response to air quality control or heating/cooling demand control where the plant only runs when required.
  
• Systems should be carefully designed to minimise pressure losses in terms of ductwork distribution as detailed below, and in careful selection of the air handling unit components.

Ductwork distribution
Considerable energy savings can be achieved through good system design, minimising motor loads. Accurate calculations of system resistance are essential for sizing and appropriate fan selection. A small increase in duct cross sectional area, as recommended in Part L2, can significantly reduce system losses and hence power requirements. However, this needs to be set against the increased cost of the ductwork. The cost premium, for ductwork and insulation, for a 10% increase in cross sectional area ranges from 10-15%. In designing to maximise energy savings, the system design should ensure that:

• The number of bends is kept to a minimum.
  
• Ducts are sized in accordance with the recommended guidelines and the use of reduced duct sizes is avoided.
  
• Duct lengths are kept to a minimum – that there are no unnecessary duct runs, and that ceiling and floor voids are used as supply and extract plena where appropriate.
  
• Fittings with a low resistance to airflow are used.
  
• Good fan inlet and outlet conditions are provided to minimise resistance loss.
  
• There is minimal air leakage from ductwork.

Fans
Fans constitute the most significant part of the energy demand in ventilation systems. Fan selection depends on the application and needs to be evaluated in terms of cost, efficiency, size and ease of installation, noise, pressure development capability and power/overloading characteristics. Project constraints may mean that the most efficient fan cannot always be selected. Whole life cost analyses may also be important to establish whether the least cost solution is a lower or higher efficiency fan. All fans should be sized as close to the actual demand as possible to minimise running and capital costs.

The fan types most commonly specified are axial flow, centrifugal (forward curved, backward curved, aerofoil bladed) and mixed flow. In general, centrifugal fans are more efficient, controllable and quieter than the alternative axial or mixed flow fan types and so are widely used. Typical efficiencies for the centrifugal range are 45-70% for forward curved, 65-85% for backward curved and 80-90% for the aerofoil blade type. High efficiency fans such as the aerofoil blade have the added benefit of lower noise levels. Use of these fans can, according to the manufacturers, result in running cost savings of up to 25% (based on installations within existing systems), providing a two year payback period.

Heat recovery
The benefits of heat recovery are dependent upon the climate and operating period, which determine the amount of waste energy that can be recovered and the amount of additional energy, fan power for example, required to drive the reconfigured system. These variables need to be considered on a case-by-case basis. However, in the UK, in the majority of cases, heat recovery options are worth considering. The three main devices used within air handling units are considered below, with a comparison provided in table 1. All options use air-to-air heat recovery and transfer sensible or total heat from the exhaust air stream to the fresh air inlet.

• Thermal wheel – this is a cylindrical drum heat exchanger which rotates slowly between supply and exhaust air streams, transferring heat from the warmer air to the cooler air. A sensible heat exchanger may recover up to 65% of sensible heat, while a hygroscopic exchanger can recover around 80% of total heat (ie sensible and latent). A small motor is required to drive the rotor, and so additional electrical energy is required. A typical pressure drop for this device would be in the region of 150 Pa.
  
• Plate heat exchanger – this consists of a number of rectangular, parallel plates. Warm and cold air flows in alternate channels between the plates, with heat transfer taking place between them. Typical efficiencies in the region of 30-70% can be expected, depending on the spacing of the plates, but is typically less than 50%. These devices require no motive power. The pressure drop for the device is similar to that of the thermal wheel.
  
• Run-around coils – these comprise finned tube copper coils positioned in the supply and exhaust air streams. Water is used as the heat conveying medium, and is pumped from the warm coil to the cold coil. Typical efficiencies of 45-65% can be expected, depending on the number and spacing of the coil rows and prevailing temperatures. Additional electrical energy is required to operate the pump. The typical pressure drop for this device can vary between 200-280 Pa depending on the number of tube rows.

Run-around coils result in a greater pressure drop than either thermal wheels or plate heat exchangers and the fan load required is greater, however differences in coil construction can result in improvements in operational efficiency. Methods of improving efficiencies include:

• The material thickness and efficiency of heat transfer, of the tubes and fins.
  
• Use of profiled fins, which create turbulence within the body of the coil, increasing contact time with the air.
  
• The spacing of the fins, as this affects the number of coil rows and heat transfer characteristics.
  
• The positioning of the coil connections, in relation to the water flow throughout the coil.

Motors and drives
Careful selection of the electric motor and drive is also important in increasing the efficiency of the ventilation system, as part of the overall fan selection. The following key issues need to be addressed when selecting motors and drives:

• Sizing of electric motors. Motors are often rated well above the power levels at which they operate. Significant reductions in efficiency occur at 25% or less of full load. Avoiding oversizing will result in both capital and running cost savings.
  
• Use of higher efficiency motors. These can save an average of 2-3% of energy consumption compared to standard motors. These devices qualify under the ECA scheme.
  
• Use of direct drives rather than belt drives. This eliminates both belt drive energy losses and maintenance costs.
  
• Use of variable speed drives to provide both variable flow control and system regulation. Variable flow control enables fans to be operated more efficiently, as the volume and hence fan power requirement, decreases as the system load reduces. Regulation using variable speed drives rather than impeller or pulley changes is fast and accurate, and involves no increase in system resistance.

As a result, the additional cost can often be justified for this purpose alone. Fan manufacturers are now increasingly offering built in variable speed drives rather than as separate items, with significant cost savings. Variable speed drives also qualify under the ECA scheme.

Case study
The case study illustrates the potential benefits of incorporating the energy efficiency measures discussed above into the design of a system.

For two representative air volumes, the typical relative reduction in running cost and corresponding increase in capital cost is provided for those elements suitable for incorporation within an air handling unit (table 2). The costs are for the supply of the plant only and do not include for installation. It is assumed that the air handling unit contains the following components: supply fan, exhaust fan, heating and cooling coils, and bag and panel filters. All payback periods have been calculated on a discounted basis at a rate of 8%.

The figures for the heat recovery options are based on standard office hours and take into account boiler, chiller, fan and pump consumption. The performance of the thermal wheel is based on sensible heat exchange only.

The figures for the variable speed drives are based on a simple representative example, of running the fans at two thirds capacity for six months of the year, and illustrate the kinds of savings that can be achieved through use of this technology. These figures represent the saving in fan consumption only, and do not take into account chiller, boiler or pump consumption figures.

Table 3 provides an illustration of the likely reduction in running costs in response to a decrease in the system resistance for the same air handling unit configuration and volumes. This demonstrates the importance of accurate system resistance calculations in reducing running costs. The saving in capital cost is based on a decrease in motor size, which may or may not apply depending on the motor selection.

In conclusion, the typical payback periods illustrate that all the options are worthwhile, and should certainly be considered for use on the majority of projects, particularly where large air volumes are being used, as the payback periods generally reduce as the volume increases. The heat recovery options have longer payback periods than the other features, and require the largest capital investment. For this reason, analysis on a whole life cost basis should be considered to fully demonstrate the benefits.

Acknowledgements
Mott Green and Wall would like to thank Woods Air Movement, Kiloheat Fans Ltd and Genesys Environmental for their assistance in the preparation of this article.