How controlling boiler plant can cut fuel bills by 40%
Common-sense approaches to energy-efficient boiler control — Julian Miller.
Common-sense methods of controlling boiler plant are frequently not exploited. Julian Miller explains how energy savings of at least 20% and as much as 40% can be achieved.Every year, according to the Building Research Establishment, industry and commerce lose millions of pounds because 90% of commercial buildings have inadequate heating and ventilation controls. This is because buildings are predominantly set up for the comfort of occupants, rather than energy efficiency. However, these two objectives are not mutually exclusive. It is possible to achieve energy savings without affecting the delivery of building services. The central solution to achieving this is the building—energy-management system (BEMS). BEMSs are often used as glorified time clocks, merely to turn plant such as boilers on and off. Part of the solution
The BEMS is therefore often considered the problem that causes energy oversupply. In fact, the BEMS should be seen as part of the solution to achieving optimum energy efficiency. A great deal of information is available from the BEMS, so it is simply a question of finding this information and using it to the best advantage. Once that information is collated, a better understanding of what is happening in the building and where the building’s inefficiencies are can be gained. This data can then be interpreted to provide solutions. Long-term energy consumption can then also be monitored to ensure that this energy efficiency initiative is maintained. Without gaining the necessary information and then programming it properly, a BEMS will remain a glorified time clock. Monitoring
Initially to determine whether the BEMS is working efficiently, key temperatures at one-minute intervals should be measured over a given time (say 24 hours). An appropriate control strategy can then be implemented directly to the system to deliver savings. The first main principle, and possibly most effective method of energy management is to supply heat only when there is demand for it. This may sound obvious, but it is often the root cause of much inefficiency. Demand sequencing of boilers and chillers is often a more significant energy-efficiency measure than insulation. Demand sequencing ensures that only the required number of boilers are firing to meet demand, thereby minimising consumption. This is only possible when the BEMS is set up properly. Most BEMSs send a signal to boilers enabling them to run. They then run on their own internal sensors and ‘dry cycle’ — often with a minimal difference between flow and return temperatures, indicating little or no load. Temperature control
In most low- or medium-pressure hot-water systems, the typical approach to ensure that heat is always available is flow-temperature sequencing. This limits temperature output to a maximum of about 83°C. However, since heat is not always required, a more efficient method would be to ensure boilers are return-temperature sequenced. This controls boilers to a lower target return temperature. The difference between flow and return temperatures can then be increased, lengthening the period between boiler firing intervals without reducing the heat supplied. Boilers do not necessarily need to reach temperatures as high as they would with flow-temperature sequencing, and the firing cycles are reduced. Outside temperatures
As well as sequencing boilers, outside temperatures must be taken into account in the form of direct compensation. This involves clarifying the relationship between outside-air temperature and the heat required to maintain a specific space temperature. Obviously, the warmer it is outside the less heating is required. This relationship needs to be understood in mathematical terms to ensure that the compensation algorithms are identified and set correctly. Compensation should be set where appropriate, allowing only the required amount of heat into a building, eliminating more waste. Setting a target flow temperature by reference to outside air temperature allows boilers to increase activity when it is colder outside and vice versa. Similarly, where variable-temperature circuits exist, each zone valve should be set correctly to compensate for external-air temperature. A key problem in many BEMSs is that there is often a rapid rotation of boilers firing; when one is post-purging another is pre-purging. This results in boilers effectively ‘chasing’ each other on and off — a clear indication of inefficiency. The BEMS should ensure that only the appropriate number of boilers fire to match the specific demand. Chillers
Another obvious route to better energy management is to prevent heating and chilling operating in the same place at the same time. In nearly all cases, where chilling is present in a building, the heating will at some stage also be running, which results in the two fighting each other. Resolving this problem will lead to greater savings in both gas and electricity. With proper BEMS control, the energy savings that can be achieved are substantial. Research across all kinds of commercial buildings has found that there are surprisingly few places where these apparently common-sense methods are in place. Through the implementation of such methods, AEC is able to achieve at least 20%, and as much as 40% or more in annual savings on heating, in most commercial buildings. Fast payback
One example of what these measures can really achieve is provided by our work in the London Borough of Tower Hamlets. A 55% reduction in energy costs in the Hind Grove estate helped around 430 dwellings benefit from cheaper fuel bills. The annual energy expenditure dropped from £111 000 to £50 000, and payback on the project was achieved in just two months. The moral of this story is a familiar tale: you cannot manage what you cannot measure. Energy efficiency in buildings is not a black-an- white issue because buildings are so dynamic. But effectively programming the BEMS can optimise these dynamics, from an energy-efficiency point of view, to achieve substantial energy savings. Julian Miller is a director of AEC Ltd , Timbers, Picketts Lane, Nutley, East Sussex TN22 3EG.
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