The future of storage

Whilst there is an increasing need to understand which of the available Low and Zero Carbon (LZC) Technologies can work together to optimise system energy  efficiency, it is also important to go back to basics with regard to the design of the conventional part of the hot water system and consider what is the most effective combined solution. 

For many years now separation of the heating and hot water loads has been recognised as the most energy efficient way of delivering these services in commercial, industrial and public buildings. Whilst the principle was first introduced when direct gas-fired storage water heaters came onto the UK market, it proved just as effective when instantaneous or constant flow type appliances became an option for larger load and higher peak demand applications. Though these are often first choice units in the food and other process industries, they have proved a popular alternative for system designers in many types of commercial buildings.

The separation concept and the use of high recovery direct-fired water heaters found favour with architects initially because it reduces the size of plant room required due to reduced stored water requirements, especially if the system is decentralised with units installed close to the point of use.  Whichever way, the amount of sales floor area, or lettable space, can be maximised.

Heading back to stored water

It was perhaps fortuitous that the industry developed in this way because it happens to lend itself particularly well to integration of LZC technologies with today’s high efficiency condensing boilers and direct fired storage water heaters. Design engineers are once again having to satisfy the architect’s and developer’s specifications but this time in relation to energy saving and reduced carbon solutions that meet increasingly stringent building regulations and planning approvals with caveats for inclusion of 10-15% of LZC technologies. So now energy savings, running costs and carbon footprint are just as important as space premiums.  A heating system that can meet the criteria on efficiency and suitability for integration with the new and emerging LZC technologies will have a positive bearing on all these objectives.  We are therefore most likely to see a return to a higher proportion of commercial heating and water heating systems that incorporate a higher degree of hot water storage.

This is because behind every LZC technology currently available that produces heat as opposed to electricity, there is the primary requirement for stored water.

This type of energy cannot be used instantaneously, whether generated by solar thermal arrays, either via flat plate or evacuated tube collectors, by ground source or air source heat pumps, or by Combined Heat & Power (CHP) products.  It therefore has to be stored so it can be drawn upon as and when it is needed and, to date, a well insulated water storage cylinder is the preferred method.  This enables conventionally generated energy to be used on an ‘as required’ basis, to top up the temperature of the stored water during periods where demand exceeds the capability of the LZC technology.   Water storage (indirect cylinders or buffer vessels) can also assist in preventing engine and compressor based products from cycling on and off too frequently which could lead to premature component failure.

Integrating technologies

A typical example of the progression is a building that 30 years ago would have been most likely to have had two large cast iron boilers, to allow for standby operation and maintenance shut-down. This would have been oversized for the average heating and hot water load of the entire building to allow for peak differentials and would fire in summer months to heat the hot water calorifier only.

The monumental inefficiencies of these times have gradually been addressed on both the heating and hot water side by the introduction of independently fired water heaters and the more recent development of smaller, much more compact yet high output commercial boilers, which can operate in modular formats and now have the benefit of condensing technology. Increasingly, solar thermal systems are being utilised in conjunction with a boiler or a direct fired water heater and a storage cylinder to reduce primary fuel consumption and carbon emissions.  

The next logical step is to consider an LZC solution to work on each aspect of the system i.e. a low carbon heating solution with separated low carbon hot water solution.

In this scenario, using solar thermal solutions to pre-heat the cold water feed into a direct-fired storage water heater can result in a dramatic reduction in the amount of gas required to raise the temperature of water from a cold water inlet from around 10°C to a hot water outlet of 60°C.  Seasonal variations in the available irradiation mean that the contribution delivered by the solar thermal solution in the summer months is far greater than in the winter.

Sizing the solar thermal system so that during the summer there is little or no primary heating appliance operation could enable condensing storage water heaters, with gross efficiencies of up to 98%, to work in harmony with solar thermal solutions to deliver an ultra-low carbon hot water generating solution.

Cutting carbon and costs

What could be done to reduce carbon emissions and save running costs on the space heating system?  A ground source heat pump could be installed to satisfy the base thermal load, with conventional high efficiency condensing boilers providing supplementary heat during periods of peak demand.  Ground source heat pumps harness the solar energy stored in the earth from incident solar rays, which is captured through ground loop pipes buried within a trench or deep bore holes.  These heat pumps achieve the best performance when delivering low-grade heat to say, an underfloor heating system.

This type of installation would maximise performance through water outlet temperature control, enabling heating boilers to be used for the higher temperature radiator circuits only as required.

This provides a low carbon space heating solution to compliment the low carbon hot water generating solution offered through the use of direct-fired water heating and a solar thermal cold water pre-heat system.  However, it might be possible to effect further carbon footprint reductions, depending on the application.   Although ground source heat pumps are electrical appliances, they are classed as renewable technology because energy in the ground is used to initiate and sustain the operation of the refrigeration cycle.  Where a building has sufficient base thermal load to support the operation of say, a ground source heat pump and a small-scale CHP unit, the latter could generate the power needed to operate the heat pump.  This would result in a significant reduction in the overall carbon footprint of the plant room.

Similarly, it may be possible to use a CHP unit to power an air source heat pump that is pre-heating the cold water into a direct-fired water heater, as an alternative to a solar thermal solution.  This could be advantageous where, for instance, the solar thermal route is not an option because there may be insufficient roof space for the solar arrays or lack of other space with suitable south facing orientation to maximise the performance of the solar collector array.

Selection and control

From just these few examples it can be seen how a hydraulic schematic for a plant room that includes conventional fossil fuelled appliances working in harmony with suitable LZC technologies can be designed to optimise plant room performance and carbon emissions.  Caution is however essential, as the equipment needs careful selection – so that operational conflict does not occur.  As well as c
onsidered and informed equipment selection, the appropriate control of the operation and interface of a variety of heat sources is critical to the success of such installations.  A well thought out control strategy will ensure that energy efficiencies and carbon dioxide reductions are maximised with no compromise in the provision of space heating and hot water.

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