Heating in harmony
With the drive to make wider use of renewable energy sources, engineers are faced with the challenge of integrating them with each other and with conventional technologies. Geoff Lord considers some of the key factors
Without a doubt, renewables are now at the top of many sustainability agendas. With some local authorities setting carbon emission targets in the form of certain percentages, while others are enforcing the energy performance requirements dictated by the Building Regulations, there is a need to include renewables as a means of offsetting the use of less efficient fuels.
When renewables are included as part of the heating solution, the real challenge is in getting all of the systems, such as biomass, solar thermal and heat pumps to work in harmony, particularly when a renewable element – solar being an obvious example – varies on a daily, or even hourly, basis.
In the UK, solar irradiation levels fluctuate widely and can range from less than 100Wh per sqm of collection area on a cloudy day, to over 1KWh per sqm on a sunny day. What this means is that any solar heating system must be backed up by a 100 percent auxiliary heating. This auxiliary heating could be biomass boilers, which work well in conjunction with solar heating and will also enhance the renewable element of the project.
When considering renewables, the key is to decide which will work well together in a controllable fashion and which overall solution will give the best results for that project.
Solar heating, for instance, is particularly good for heating swimming pools because the pool acts as a large heat sink with no need to buy thermal storage vessels. The relatively low operating temperatures, typically between 26 and 30 degrees Celsius, are easily achieved. These low temperatures result in high collector efficiencies and solar fractions.
Additionally, most indoor pools have a large roof area which is ideal for the solar collectors.
When solar heating is used for domestic hot water (DHW) it will, on sunny days, heat all of the water to the required temperature. When less solar energy is available it may be used to pre-heat mains cold water before it is brought up to temperature by another heat source.
On larger projects, which inherently have significant shifts in demand, it is usually more cost effective to install solar equipment to pre-heat the DHW and then top it up to the required temperature using either biomass, condensing or conventional boilers. This has the effect of reducing the frequency that the solar circuit goes into stagnation, particularly during the summer months, which in turn increases the system efficiency and specific energy produced by the collector loop.
It is important to note that any pre-heated potable water storage volumes should be kept to a minimum, as these may have to be regularly pasteurised as part of the anti-legionella regime. In these 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. This also has the benefit of allowing the solar energy to be safely stored at higher temperatures.
Piping a number of non-potable thermal storage vessels in series and using diverting valves to circulate the water in the vessels will enhance stratification within the vessels. This will promote solar gain and also allow for auxiliary heating sources to be fully integrated within the heating / DHW scheme and support the solar powered system.
Another way to lessen the periods of stagnation during the summer months, when solar energy is in abundance, is to increase the angle of inclination on the collectors. This reduces the efficiency of the collectors in the summer and increases their efficiency in the winter when the sun is lower in the sky, thus having a load levelling effect.
In other schemes, a combination of these renewables with high efficiency condensing boilers may be an even better solution. Where there are high demands for hot water, for instance, the solar heating and biomass boilers may be used for DHW. In parallel, condensing boilers or heat pumps could be used to supply space heating, thus taking take full advantage of the relatively low temperatures of the water returning from the building to the heating plant, to achieve maximum efficiency.
In any of these scenarios, effective control is going to be the key to achieving the desired result which, in turn, requires an excellent understanding of the technicalities of the systems involved. Hoval has extensive experience of all of these forms of heating, and of integrating them with the assistance of sophisticated control strategies to ensure that each performs to maximum efficiency.
Manufacturers of biomass boilers build their products to a high standard, producing good efficiencies. However, to achieve optimum performance, they require premium grade fuel. Typically both wood pellet and wood chip should conform to European Specification CEN TC355/CEN TS14961.
In the case of biomass installations it is also important to consider all of the practicalities, such as the implications of the chosen fuel type for fuel storage requirements and ensuring access for delivery vehicles. It is also necessary to balance costs to achieve the optimum situation for each project. For instance, wood chips tend to cost less than wood pellets but have a lower calorific value so more need to be used to produce the same amount of heat. Wood chips also produce more ash, so this has implications for maintenance requirements.
Hoval is ideally placed to advise on all of these issues and can offer a full turnkey service from initial design through to commissioning and ongoing maintenance.
Clearly, there is a need to give due consideration to integrating renewable technologies into all building services projects. Understanding these technologies and being able to design suitable bivalent systems incorporating the right products and controls for any one particular project is where the skills of companies like Hoval can play an important role in helping property managers drive down energy-related costs while also reducing the UK’s carbon emissions.
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