Renewable technologies such as solar and biomass figure frequently in today’s designs for school heating and hot water systems. There are, however, significant challenges in integrating these newer technologies with one another and with other components in the plant room. The risk is that the carbon reductions gained through on-site generation will be lost because other parts of the system are being forced to operate outside their most efficient parameters in order to accommodate the renewable parts of the system.
The first principle is to recognise that the various low- and zero-carbon technologies need different operating temperatures to perform at their optimum efficiency. Each of these elements must be integrated without forcing any component to operate at a temperature which compromises its efficiency. For example, a condensing boiler will not actually ‘condense’ if the system temperature is too high.
One solution is to integrate the LZC (low- or zero-carbon) systems with a thermal store, combined with an LZC controller and HVAC equipment designed for all-variable speed operation.
There are two types of thermal store. A sensible heat store discharges energy via temperature changes within the vessel. Latent heat stores discharge heat at a constant temperature. The former is used more commonly in LZC integration applications whilst the latter is more suited to passive heat storage/release applications.
The thermal store typically performs three functions. First of all, it levels out supply and demand. For example, solar energy available during the day can be stored for release at night. Second, it acts as a buffer to prevent boiler cycling and ensure residual heat from a biomass combustion chamber is adequately dissipated. And finally, a correctly designed thermal store allows the vertical stratification of temperatures. Hot water rises and cold water sinks, so lower temperatures, from solar thermal or heat pumps, are fed into the bottom of the store. Medium temperatures from condensing boilers are fed into the middle of the store and higher temperatures from biomass are fed in towards the top of the store.
The choice of HVAC components is also important. Traditional equipment seldom has sufficient flexibility, but that doesn’t mean you have to resort to expensive bespoke kit. Look for mainstream products specifically designed for a part load, variable speed environment. Unlike fixed speed plant, variable speed equipment is always more efficient at part load. The latest control technologies manage this phenomenon and balance load to demand for the best wire-to-water efficiency at all times. Each component is operated automatically to meet demand whilst, at the same time, exploiting its energy-efficiency potential to the full.
Particularly effective are HVAC components in which the criteria for energy efficiency are pre-designed into the control methodologies. The renewable elements are then viewed simply as additional contributing factors to the conditions and the equipment adapts automatically around them.
For example, the plant rooms supplied by Armstrong Fluid Technology for two schools in Sunderland, Academy 360 and Red House Academy, incorporate 350 to 500kW biomass boilers, fuelled by wood pellets. Heat is also generated by solar panels. Armstrong variable speed booster sets and pumps distribute hot and cold water supplies throughout each site. By utilising a number of high-efficiency variable speed components and by employing advanced controls technology, the system design ensures that energy-efficiency performance outperforms standard school HVAC installations by a significant margin.
Steve Cooper is director, sustainable design, with Armstrong Fluid Technologies
