Renewables and fossil fuels in harmony
Increasing pressure to use low-carbon technologies means that boilers are often one of a range of heat sources installed in a building. Ian Dagley discusses the implications of this trend.
Boilers are still first choice of heat source for space heating and hot water in most projects but are increasingly being required to work in harmony with, or as back up to, other heat sources. Obvious examples include solar thermal systems and heat pumps.
When specifying a heating system, therefore, choosing and configuring the right boiler is now only part of the equation in many cases. Frequently there will be an imperative to design a fully integrated system that gives priority to low-carbon heat sources and makes optimum use of conventional boilers as well.
When renewables are included as part of the heating solution, the real challenge is in getting all the systems to work in harmony, particularly when the renewable element (as with solar thermal), varies on a daily or even hourly basis. In the UK, for example, solar irradiation levels fluctuate widely and can range from less than 100 Wh/m2 of collection area on a cloudy day to over 1000 Wh/mz on a sunny day. What this means is that any solar heating system must be backed up by 100% auxiliary heating. Such auxiliary heating could be biomass boilers, which work well with solar heating and will also enhance the renewable element of the project.
At York High School, for example, the swimming pool is heated by two 450 kW wood-pellet biomass boilers and 80 kW of solar-thermal heating systems (believed to be the largest solar thermal array in the country). The solar thermal system will meet all the swimming pool’s heating load during the summer, with back up from the boilers at other times of the year.
A further three 450 kW wood-pellet boilers have been installed for space heating and hot water in the general school buildings, achieving a very high renewables element for the project.
Where a large base heating load such as a swimming pool is not available, solar heating is more commonly used for domestic hot water (DHW). Again, in many cases, the solar energy will be sufficient to meet all hot water requirements in the summer and require top up in the winter. However, with larger projects with their inherently significant shifts in demand, it is usually more cost effective to install solar equipment to pre-heat the DHW and then raise it to the required temperature using a biomass or conventional boiler.
Not only does this approach keep the capital cost down, but it also reduces the frequency that the solar circuit goes into stagnation (periods when no energy is being removed from the collectors by the solar fluid) during the summer. This in turn increases the system efficiency and specific energy produced by the collector loop.
It is also important to note that the volume of pre-heated potable water stored should be kept to a minimum, as it may have to be regularly pasteurised as part of the anti-legionella regime. In such circumstances it is beneficial to store the solar energy in a thermal storage vessel and pre-heat the cold feed water through a suitable heat exchanger.
Given the potential for combining and integrating different heat sources, this approach can also ensure that each is used to optimum effect. During renovation of a Grade II listed hall in Northumberland, for instance, solar heating and ground-source heat pumps have been combined with an oil-fired boiler — all feeding to a thermal storage vessel. The 1200 litre vessel, with an internal 300 litre stainless steel sphere for DHW, can receive hot water from up to three sources.
Through a control system these vessels operate on a cascading principle utilising, when available, solar followed by tandem heat pumps and, ultimately, the boiler for peak loads. Making effective use of thermal stratification, the vessel and controls ensure that water at the top of the vessel is at the required temperature and that DHW is maintained at 55ºC.
In other situations, optimum performance may be achieved by combining biomass boilers with gas-fired condensing boilers. For example, where a low-temperature heating system is used the greatest energy efficiency may be achieved by enabling the condensing boiler(s) to exploit low return-water temperatures, while the biomass boiler(s) provides DHW.
These examples illustrate that the design of heating systems is getting more complex and that specifiers need to give considerable thought to all the available options and how they can be combined in the most energy-efficient fashion. Very often, achieving the required level of integration will necessitate sophisticated controls that are able to achieve the desired harmonious performance — music to the ears of everyone concerned about the environment.
Ian Dagley is sales director of Hoval.
