A newly improved custody suite has been commissioned for the Thames Valley Police at Maidenhead. It has 10 cells and features an innovative lighting, heating and ventilation system which is the subject of this piece. Our editor is speaking to Mr Alan Bowden who is a Mechanical Consulting Engineer from the firm Turner, Wright & Partners, who are based in Maidenhead and who were responsible for the design of the system.

I’ve come today to gather information about the energy saving innovations that have been built into this improved custody suite, which I understand, your firm designed. Can we start with the heating system I’ve heard so much about. What is so novel about that?
Alan Bowden (AB) It’s novel in the sense that we are recovering heat from areas where we don’t want it and directing it into places where we do. Heat recovery has been around in some form or other for quite a few years and mainly applied to ventilation systems - that is as air is taken out of the building, the heat content is recovered and transferred to the air coming into the building and not wasted. But at High Wycombe we are using a new system. We take heat from a room (cooling) and transfer it into another room where there is a demand so rather than doing it via the ventilation system (which is referred to as a global system) we are doing it room by room.

Without wasting the energy generating the heat in the first place, clever! Can you quantify the heat savings typically available?
AB Heat is generated by an air-to-air heat pump and this installation at High Wycombe will save about 28,260 giga joules a year (that works out to be about £3,000 a year) expressed as the cost of electricity. In addition to recovering and reusing energy, we are collecting heat from the sun which, of course, is a sustainable resource. Electricity is used to rotate the motors of the heat pump which is moving the air, and extracting the heat/cooling it contains, one way or the other depending on the demand. There is an ‘inverter’ drive on the heat pump which adjusts the speed of the motor to the demand. Rooms and cells have to be available 24/7, but of course they are not occupied all the time. There is a microwave sensor fitted into the ceiling of each room and in the locking mechanism of each cell, that detects occupation or not and switches lights on or off and adjusts the heating/cooling down or up (22˚C to 20˚C, or 22˚C to 24˚C) accordingly. There is an over-ride for each room, whereby the heating can be adjusted. Quite a lot of the rooms are unoccupied for longish periods and by lowering the temperature in these by just 2 degrees when not in use, the computer model predicts a saving of £2,500 per year.  If during occupation the temperature has been amended locally, it will return to the default settings when the sensors detect no occupation.

But you have to set against that the capital cost of the installation. How much is that?
AB £2,500, so one year of savings covers the capital cost.

Any more innovations you can tell us about?
AB We also recover heat from the ventilation system. Heat generated by equipment and the human body is taken away by the ventilation system, the heat content extracted and used to heat the fresh air coming in. The cost of that operation depends on the outside air temperature and in the winter months, (say) minus 5˚C to 21˚C is a steep temperature gradient and would be expensive. By extracting heat from the outgoing air to warm that coming in, 80% heat transfer can be achieved, making the system viable.

Have you got an equally efficient method of dealing with lighting?
AB As you may appreciate the general cell areas need to have a certain minimum level of lighting in order to function. But cells are not in use all the time so occupational sensors are fitted to the corridors which bring up the lights when someone enters. Once the detainee are settled the lights in the corridors are dimmed.

Thank you for taking time out to speak with the Review.