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Technical answers - geothermal energy, energy audits and performance ratingsJanuary 2011

Clare Sinclair provides the answers to technical questions posed by BSRIA Members. Issues covered include geothermal energy, condensing boilers and VAV boxes.

Wood Pellet Heating Systems is available on library loan to BSRIA Members

No hot rock renewables?

Q. Why doesn't BSRIA's publication BG 1/2008 Illustrated Guide to Renewable Technologies cover geothermal energy?

Geothermal energy is extracted from heat generated deep in the Earth. When extracted, this heat can be used to heat buildings directly or to drive electricity turbines. In the UK its application is limited to a few areas where geothermal heat, in the form of hot springs, hydrothermal circulation, or near proximity to magma (hot rocks), are close enough to be accessible by boreholes. Areas where this is economically possible are around Southampton and Bath. The use of geothermal energy is unlikely to become mainstream in the UK unless the cost of drilling very deep boreholes reduces.

Condensing boilers

Q. Under what conditions will a condensing boiler actually condense?

Condensing boilers are considered more efficient because they recover some of the latent heat held in the water vapour that would otherwise be emitted in the flue gases. However, gas will only condense on the heat exchanger if the return water temperature is below 57ºC for gas-fired boilers or 47ºC for oil-fired boilers. The lower the return water temperature, the greater the efficiency, up to a maximum of about 30ºC. There's nothing to prevent the flow temperature being much higher than the return temperature, as long as the system is designed to achieve a large temperature difference between the flow and return.

VAV pressures

Q. What is the difference between a pressure-dependent VAV box and a pressure-independent VAV box?

A VAV (variable air volume) air-conditioning system is one which satisfies the cooling requirements of multiple zones by delivering variable quantities of air, at a constant temperature, to each zone. This is done by changing the damper position in the VAV box in each zone. A pressure-dependent VAV box uses a simple control relationship between space temperature and damper position. So if the pressure in the duct changes such as other VAV boxes elsewhere in the building changing positions, the temperature in the room will change and the damper will adjust to compensate, after a time delay.

A pressure-independent VAV box constantly monitors the air flow, and adjust it in response to space temperature. When the pressure in the duct changes, the damper will adjust to maintain the same flow without waiting for the temperature in the room to change.

Energy audits

Q. What is the time required to undertake an energy audit of an air conditioning system?

This depends on the extent of the system installed. Section 1.7 of CIBSE TM 44 covers this in more detail saying: (for a) "simple 12 kW split packaged unit, the process should take only 2 hours". However, TM 44 also says it is "doubtful whether a larger system that includes indoor and outdoor cooling plant, AHUs and zone controls could be completed in much less than a day, and substantial installations could take longer". Some tasks such as examining AHUs or ducts may "need to be undertaken outside normal working hours, and must be carried out in collaboration with the building owner or manager, and subject to a proper risk assessment".

Performance ratings

Q. How are weather louvre performance ratings assessed?

Testing of weather louvres is covered by EN 13030:2001. This covers two tests: a water penetration test and a pressure loss test. Test samples should be as close to 1000 mm x 1000 mm as the blade increment allows, to give a core area (the opening in the front face of the louvre) as close to 1 m2 as possible.

In the water penetration tests, the external conditions must be constant throughout, with a horizontal air velocity of 13 m/s at 1 m in front of the test sample. Water is sprayed into the airstream at approximately 100 l/h - a rate that would lead to a penetration rate of 75 l/h through a 1 m x 1 m hole. The excess is overspray and evaporation.

With the test sample in place, the water that penetrates is captured and measured, and compared to the rate without a louvre. Therefore if up to 0.75 l/h penetrates a 1 m2 louvre, the louvre is better than 99 per cent effective. While the external conditions remain constant, the rate at which air is drawn through the louvre sample is increased in stages, up to a maximum face velocity of 3.5 m/s. Hence a louvre will have a range of weather ratings over the tested velocity range through the louvre.

The second test covers the air flow performance of a louvre. Air is drawn through the sample at a range of velocities, with the pressure drop noted at each point. This allows a graph of velocity versus pressure drop to be produced. The actual air velocity at each pressure is also compared with the volume that would be expected from a theoretical perfect louvre. This gives a coefficient of entry figure that describes a unit airflow performance, thereby allowing simple comparisons between units.

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