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Where there is a high demand for domesc hot water, appropriate system design will enable opmum use of low carbon heang technologies. Ian Dagley, general manager of Hoval, considers some of the opons


hilst most commercial buildings tend to have a higher demand for space heating than for


domestic hot water (DHW), there are certain types of facility where this situation is reversed. Obvious examples include hotels and other hospitality venues, as well as leisure facilities. In such situations the choice of heating plant and the control strategy applied will be critical in delivering optimum performance and efficiency, and taking full advantage of low carbon heating technologies such as solar thermal and combined heat and power (CHP).


This can be illustrated by considering the opportunities for including solar thermal in the heating mix and may require a ‘re-think’ of traditional strategies. For example, many such mixed heating systems have been designed to use renewables as a supplementary ‘top up’ source to the traditional heating technologies. Inevitably, this imposes limitations on the use of the renewable element and, in most cases, is not the optimum configuration. A better solution when solar thermal heat sources are included in the system is to use the solar energy as the pre- heat source for domestic hot water (DHW) and then supplement from other heat sources.


In such a pre-heat configuration, a mild steel vessel is also incorporated into the system, solely for storing the water heated by the solar panels and pre- heating the DHW via a plate heat exchanger. A clear benefit of this arrangement is that it isolates the solar- heated water from the potable DHW supply so that there is no risk of legionella. This means the water in the mild steel vessel does not require daily pasteurisation, thereby reducing energy consumption. Other anti-legionella measures for stored DHW will, of course, need to continue as usual.


uSolar energy can be used as the preheat source for domesc hot water (DHW) and then supplemented with other heat sources in the heang system


If high storage volumes are required, a number of non-potable thermal storage vessels can be piped in series, using diverting valves to circulate the water and enhance stratification. This will promote solar gain and also allow other renewable heat sources, such as biomass boilers or heat pumps, to be fully integrated within the scheme and support the solar thermal system. A further benefit of this arrangement is that it makes use of the solar irradiation that is available even on cloudy days. Even if the hot water in the mild steel vessel only reaches 35°C this will help to reduce the energy consumption of the


BOILERS & HOT WATER Catering for high DHW demands


uModern controllers now need a ‘wider remit’, insofar as they can take eecve control of a range of heat sources, with the ability to handle single units or operate cascades of heat sources


other heat sources. As long as the stored water is at least 6°C above the temperature of the incoming mains water, there is an opportunity to achieve a worthwhile heat transfer. On days when solar energy is plentiful, the temperature of the water can be allowed to reach very high temperatures, up to 90°C, before the system pumps are switched off, without any risk of scalding. Using the solar energy to pre-heat DHW also makes it very straightforward to retrofit solar panels to an existing heating system, thereby enhancing the business case for investing in this renewable heat source.


CHP and heat networks


With the introduction of small-scale combined heat and power (CHP) to the market, many hospitality and leisure venues are now able to take advantage of this technology as part of their heating mix. Typically, where CHP is used it will be part of a mixed heating system that also incorporates other heat sources.


A familiar configuration is to use gas- fired CHP alongside gas-fired condensing boilers. Here, the CHP will usually be used to meet base heat loads with the gas boilers providing a top-up at times of peak demand. This arrangement needs to maximise the run-times of the CHP, so it’s important that the base heat load is sufficient for the CHP to run for at least three hours each time it fires. Here, again, inclusion of thermal storage capacity can be useful as it enables the CHP to keep running once the building’s immediate heat loads have been satisfied, thereby increasing the amount of onsite power generation.


Taking and maintaining control


The situations described above are just some examples of how a high demand for DHW can be met with a mix of heat sources. In all such cases, efficient


14 BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2019


control is essential. Experience has shown that trying to control the multiple heat sources with different controllers is extremely difficult and will usually compromise overall performance. Consequently, modern controllers now need a ‘wider remit’, insofar as they can take effective control of a range of heat sources, with the ability to handle single units or operate cascades of heat sources. For example, it is now possible to install a single controller that will control CHP, boilers, heat pumps and solar thermal and other heat sources that may become available in the future. Connectivity through the Internet is now also an expectation of many building operators, so that the system can be monitored remotely from a computer, tablet or smartphone, with alerts for routine or reactive


maintenance. Similarly, integration with a building management system using OPC UA, ModBus or KNX interfaces, along with ‘smart grid’ readiness, are rapidly becoming ‘de rigueur’. Equally, controllers need to make it easy to re-commission the system to reflect changing DHW and space heating loads through the life of the building – perhaps due to changes in usage or improvements to the thermal performance of the building fabric. In all these cases the control strategy needs to be updated accordingly and this is a far more straightforward process when using a control system that is easy to re-configure and re-commission.


Summary


When meeting the demands of buildings that have higher heat loads for DHW than space heating, delivering the most efficient solution requires a combination of optimum system design and efficient control. In that way, the building operator gets the


environmental and cost advantages of low carbon heat without compromising on the service provided to the building’s users.


www.hoval.co.uk Read the latest at: www.bsee.co.uk


‘


With the


introducon of smallscale combined heat and power (CHP) to the market, many hospitality and leisure venues are now able to take advantage of this technology


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