Putting the E back into BMS
Rather than just control heating and cooling to achieve comfort levels, Gary Perry advocates also using energy efficiently — and has lots of suggestions
There are many easy and 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 before asking three simple questions. If it is on, how can I switch it off? How can I turn it down? How can I make it more efficient?
The key to asking these questions 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 plant that is more efficient but not properly controlled.
Taking control and re-programming a building- management system into a building and energy-management system 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 operated only when needed. Use a 7-day programme and ensure optimal settings are chosen for the different days. If the building is occupied for only 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 on the external temperature. Introducing a programme that monitors the external temperature can ensure the warm-up period is optimised. A higher temperature outside would mean a shorter warm-up period; similarly, 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 operate only when an area calls for it — and 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 – which 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? Program 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 1 K and increasing the cooling setpoint by 1 K both result in a 10% energy-consumption saving.
Linking to the external temperature also allows adjustments in setpoints; higher ambient temperatures allow for higher cooling setpoints and vice versa for heating.
Isolation. When the external temperature reaches 16°C, the building will not normally need heating, so the heating system should be locked out. Similarly, when the ambient temperature drops below 13°C, 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.
Weather compensation. The flow temperature of an LPHW heating system can be varied according to the outdoor temperature. As the outdoor temperature rises, the 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 temperature of the flow and return water. As the temperature of the return water decreases, the flow temperature can be increased.
Free cooling. The ambient temperature in the UK is below 15°C 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 1 K below the temperature of the return water will provide cooling, and 3 to 5 K will result in the compressors not being needed at all.
Anti-cycling. Heating and cooling systems working close to their setpoints 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 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.
Gary Perry is director of sustainability, energy and projects at Norland Managed Projects.