Modern Building Services
Putting the 'E' back into BMS
By Gary Perry, director of sustainability, energy and projects at Norland Managed Projects. Published in Modern Building Services in October 2011. Click here to view the published article.
There are many easy, quick, steps that a facilities manager can take to improve the energy performance of buildings. The very first, and possibly most obvious, is to walk around the building and review all the appliances, equipment, plant, and other systems, and ask three simple questions: if it is on, how can I switch it off, how can I turn it down, and how can I make it more efficient?
Key to this is the effective control of plant and lighting. Proper control of older and less efficient plant can often be a better solution than installing newer, more efficient plant which is not properly controlled. Taking control and re-programming a BMS into a BEMS is probably the most important energy efficiency step a facilities manager could take, and it does not require an enormous outlay. There are several elements to be considered, and each can lead to significant savings:
Basic time control Ensure the plant is only operated when needed. Use a seven day programme and ensure optimal settings are chosen for the different days. If the building is only occupied five days a week, then keep settings at a minimum at the weekends. Also, take into account holidays such as Christmas and bank holidays.
Optimum start/stop The length of the warm-up period for a building depends upon the external ambient temperature. Introducing a programme which monitors the external temperature can ensure the warm-up period is optimised. A warmer ambient temperature outside would mean a shorter warm-up period. Similarly, if warmer outside, the system can switch off plant earlier at the end of the day so the building runs on residual heat, reducing run times. The same approach applies to cooling in the summer.
'On-demand’ Introducing area “on-demand” controls ensures that heating or cooling only operate when an area calls for it, and it will only happen if the area is above a set point temperature for heating, or below a set point for cooling. To enhance this, all heating and cooling equipment in an area should be grouped and controlled together – it will also prevent them from fighting each other.
Sequencing Plant should be sized to come on sequentially. Why initiate the entire heating or cooling capacity when only a proportion would suffice? Programme the system to start slowly and build up only to the capacity required to maintain the required temperature.
Deadband The difference between the heating and cooling setpoints, known as the deadband, should be as wide as possible and must take into account the external temperature. Reducing the heating setpoint by one degree and increasing the cooling heating point by one degree both result in a 10 per cent energy consumption saving.
Linking to the external ambient temperature also allows adjustments in set points – higher ambient temperatures allow for higher cooling set points, and vice versa for heating.
Isolation When the external temperature reaches 16oC, the building will not normally need heating, so the heating system should be locked out. Similarly, when the ambient temperature drops below 13oC, some buildings will not need cooling, so the cooling system can be locked out (in modern buildings with effective insulation and with high internal heat gains - the cooling load is not so dependent upon external ambient temperatures and the cooling cannot be locked out).
Compensating The flow temperature of a LPHW heating system can be varied with external ambient temperatures. As the temperature rises, heating requirement falls, so the flow temperature can be reduced. Similarly with cooling, as the ambient temperature falls, cooling need will be reduced, and this will be reflected in the difference between the flow and return water temperature. As the return water temperature decreases, the flow temperature can be increased.
Free cooling The ambient temperature in the UK is below 15oC for more than 75% of the year providing huge opportunities for free cooling. It can be as simple as introducing cool air into the building overnight or early in the morning. Another approach would be to introduce a free (dry air) cooler, with return water automatically diverted through it before returning to the chiller. An ambient temperature of just 1oC below the temperature of the return water will provide cooling, and 3oC to 5oC will result in the compressors not being needed at all.
Anti-cycling When heating and cooling systems are working close to their set points, they tend to operate in short bursts, which is very inefficient. Also, in autumn and spring, it is not unknown for heating and cooling to be needed intermittently. Anti-cycling programming will artificially keep the systems running to improve efficiencies and reduce the demand for re-starts.
Calibration It is very important to ensure that the sensors of the BEMS are calibrated regularly, as the BEMS controls according to its numerous sensor readings. Inaccurate sensors will probably lead to higher energy consumption.
