1       Control strategies

 

1.1       General principles about occupancy and control schemes

Regulation and control systems have the difficult task to maximise comfort for the building occupants while minimising energy consumption. There is no conflict between those two functions, but an issue of complexity: simple systems will most often focus on user comfort and leave room for energy efficiency improvements while more complex and elaborated systems can both increase comfort and energy efficiency.

It is important to notice that comfort is linked to building occupancy.  Outside the occupation hours, the building comfort does not matter. However, many optimised systems will stay operational during occupant's absence and waist energy this way. On the other hand, also outside the occupation hours certain temperature and humidity limits are imposed by the building itself or by equipment or stored goods.

The different aspects of indoor user comfort have been defined in Part II – Chapter 1 – Indoor user comfort. Regulation and control have to be adapted to the designed systems in order to satisfy all aspects of user comfort, taken into account all possible interactions, occupant's intervention and variable boundary conditions. The following section elaborates some principles of regulation and control for different systems.

 

1.2       Developing building adapted control strategies

The following paragraphs will briefly mention some options and attention points for creating building adapted and energy efficient control strategies. It is not the intention in the scope of this document to give a complete overview or to discuss all aspects of these points.

1.2.1    Heating

The two most important heating parameters are the zone temperature setpoint and the heating schedule. Setting the temperature setpoint too low will lead to comfort problems, while too high values lead to energy waste. In situations with low radiation temperatures (uninsulated buildings with large (single) glazed window areas) the air temperature needs to be increased to reach the same comfort temperature as with high radiation temperatures. Normal (air temperature) setpoints are in the range of 19°C to 23°C. The savings of one degree setpoint decrease can be estimated if the average winter temperature is known. In Brussels (6°C average winter temperature), this is 1/(21-6)=7%.

The heating schedule needs to ensure comfort during the occupation hours. Setting the schedule to the occupation hours will often lead to discomfort because of the inertia of the building and the heating system. In the morning, the heating needs to be switched on before the arrival of the occupants, and on Monday mornings even earlier (if the heating was off in the weekends). In the evening, the heating can often be switched off before the occupants leave. During the off-period the setpoint can be reduced (night set-back) or the heating can be completely off. The night set back temperature should be sufficiently low and the pre-heating period needs to be adapted to the available heating power in order to reach the setpoint again when the occupants arrive. Advanced regulations can change the start-up time in function of inside and outside temperature.

Even in high inertia historical buildings, night set back or on/off regulation is advised. It is often heard that night set back does not save energy in heavy buildings, but that is not true. The heat loss of a building is depending on the temperature difference between inside and outside, so night set back will reduce the heat demand in all cases. The extra energy needed during start-up is always lower than the energy savings during the set back period. If the heat emission system is very slow (floor or slab heating), night set back is very difficult to realise. In badly insulated buildings, those systems need to be designed with care.

To have an optimised temperature control in multi-zone buildings, individual regulations per zone are required. Thus, thermostatic valves or individual thermostats (fan coil units) are always advised. On the other hand, it is noticed that giving the occupants control over the set temperature in different zones (by different room thermostats) often leads to inefficiencies and has to be avoided.

Low emission temperatures are required when integrating solar thermal systems or heat pumps in the heating concept. Variable emission temperatures (depending on outside temperature) lead to smoother regulations and even higher efficiencies, not only in the case of renewable energy systems. Boilers, specifically condensing boilers, have increasing efficiencies with lower water temperatures. 

The control of different heat sources that are combined into one concept depends on the nature of the heat sources and the design. Cascade connections are often used. Attention should be paid to the hydraulic isolation of the idle equipment to prevent heat losses (specifically for boilers).

Heat storage of any kind can be foreseen and controlled in different ways: to reduce the on-off cycling, to reduce the needed installed power or to reduce operating costs (e.g. operating heat pumps at night tariff period).

The auxiliary electricity consumption can not be neglected. Variable flowrate has to be studied and applied where possible to reduce electricity consumptions of pumps. 

1.2.2    Cooling

The control of cooling is very similar to heating. First, attention needs to be paid to the temperature setpoint and the time schedule for the cooling system. Temperature setpoints between 24°C and 28°C seem reasonable both from comfort and energy perspective and depending on climate, building and occupant's activity. The savings of one degree setpoint increase are more difficult to estimate than for heating because of the higher impact of solar and internal gains, but are normally in the range of 15%-30%. An outdoor dependent temperature setpoint will maximise both comfort and energy savings. As for heating, the cooling schedule needs to be adapted to the occupancy. Pre-cooling the building is often not needed as the cooling loads are zero or small in the morning and normally the cooling can be shut down somewhat before the last occupants leave.

Cold storage of any kind (building integrated, water or ice) can be foreseen and controlled in different ways: to reduce the on-off cycling, to reduce the needed installed power, to reduce operating costs (e.g. operating the chiller at night tariff period) or to improve the efficiency (higher condensing temperatures at night).

Water condensing systems can be equipped with a bypass to provide so-called free chilling during intermediate seasons. The control of the free chilling mode is dependent on the ambient temperature (and humidity for cooling towers) and the needed chilled water temperature.

A load dependent chilled water temperature will increase the efficiency of cold production. Alternatively, variable flowrates will reduce the auxiliary electricity consumption of pumps.

1.2.3    Ventilation

Ventilation systems provide fresh air and can be controlled based on time schedule, temperature, humidity or CO2-concentration. More complex systems will use a combination of these parameters. 

A time schedule will normally be well in line with the foreseen occupancy schedule. Pre-ventilation is normally not needed, post ventilation can be advised to remove odours, CO2 and humidity after the last occupants leave. Ventilation systems with heating and/or cooling functionalities and variable flow rate will generally be controlled on CO2 first. The flow rate will only be increased if the zone temperature moves away from the setpoint. Zone temperatures have been discussed before, the pulsion air temperature setpoint depends on the type of grids and flowrate.  Lower limits are generally not below 16°C, upper limits not above 50°C. Normally, humidity levels between 30% and 70% are acceptable. See also the European norms EN 13465 and EN 13779.

1.2.4    Lighting

Lighting can be controlled (semi)automatically (centrally or locally) or only manually. Presence detections are decentralised by default, while daylight dependent dimming can be locally (on each fixture) or centrally (per room, per façade). Large buildings often have centralised on-off or at least off-control (on by the users). Most often, this centralised off-control is based on a time schedule and can be overruled by occupants that would still be present.

1.2.5    Solar shading

The control of solar shading devices can be manual or automated (locally or centrally). An example of locally automated solar protection control can be found in the Berlaymont building in Brussels, the seat of the European Commission. The building is equipped with glass louvers, automated in sectors of 6m width by 3.3m height [COL08].  Each sector is individually controlled based on solar radiation. When the louvers are in horizontal position they reflect the daylight to the (white) ceilings [UCL08].

Figure 88: Glass louvres in the Berlaymont building

Figure 89: Operation of the solar protection in function of the solar radiation

1.2.6    Hot water

In storage based hot water production, a sensor in the tank gives a signal to the heat production when the temperature is too low. In direct production systems, a flow and/or temperature sensor on the hot water duct regulates the heating power. Although the hot water use temperature is often lower than 45°C, setpoints are usually in the range of 50°C-65°C. Storage systems generally have higher setpoints to have a higher capacity and to decrease the legionella destruction time. In large systems, there is mostly a distribution system with recirculation. Heat losses in those distributions systems can be extremely high, values of 200% of the hot water demand have been measured. If the temperature in part of these loops is lower than 50°C, there is a risk of legionella build-up. Energy savings can be realised by lowering temperature setpoints or using time schedules in combination with daily or weekly thermal desinfection. Local regulations considering legionella need to be taken into account. 

 

1.3       Building management systems

A Building Management System (BMS) is a computer-based control system installed in buildings that controls and monitors the building’s mechanical and electrical equipment such as ventilation, lighting, power systems, fire systems, and security systems. A BMS consists of software and hardware; the software program, usually configured in a hierarchical manner, can be proprietary using such protocols as C-bus, Profibus, etc. However more and more vendors are producing BMSs using Internet protocols and open standards like DeviceNet, SOAP, XML, BACnet, Lon and Modbus.

A BMS is more typical in a large building. Its core function is to manage the environment temperature, carbon dioxide level and humidity within a building. As a core function in most BMS systems, it controls the production, distribution and emission of heat and cold and the supply and condition (temperature, humidity and CO2-level) of fresh air. A key secondary function is to monitor the building and system state, the energy consumption and the occupant's comfort. Normally, the building operator can change system parameters via the BMS in order to have an optimised control system.

 

Systems linked to a BMS typically represent 40% of a building's energy usage; if lighting is included this number approaches 70%, the rest is occupant's equipment like computers, printers, etc. BMS systems are a critical component to managing energy demand.

External Links

§  http://energie.wallonie.be/energieplus/script.htm

§  http://en.wikipedia.org/wiki/Building_Management_System

 

1.4       References

[UCL08]          http://www-energie.arch.ucl.ac.be/eclairage/exemple.htm

[COL08]          http://www.coltgroup.com/projects/public-buildings/berlaymont/img_berlaymont_6_big.html