Building the case for hybrid solutions

Hybrid systems that combine renewable technologies with traditional boiler systems offer a reliable route to delivering energy efficiency in commercial buildings, argues John Bailey of Vaillant.

In recent years, we have witnessed an increased appetite for renewable technologies. Driven to varying degrees by technological advances and improved levels of awareness, a desire to be more environmentally responsible, and a need to realise financial savings in a difficult economy, more people have accepted that renewables aren’t a fad for the tree-huggers, but a genuine alternative.

Commercial boilers can be teamed up with sources of renewable energy such as heat pumps and solar thermal as hybrid systems to help meet carbon-reduction targets.

In the commercial world, interest and uptake is being driven by updated Building Regulations and various legislative demands aimed at reducing the carbon footprint of commercial buildings. Incentives such as the Renewable Heat Incentive (RHI) and promises of a ‘commercial green deal’ have perhaps spiked an interest, but I believe this is a market segment being driven by necessity over financial gain.

Delivering energy-efficiency savings in the commercial world is considerably more complex than it is with domestic properties. The mantra of ‘fabric first’ is particularly challenging to accomplish. Substantially improving the fabric of manufacturing or warehouse spaces, or older properties with complex usage such as hotels, schools and hospitals can be logistically and financially unachievable.

For some facilities managers and landlords, this has been a barrier to the adoption of renewable technologies. Until relatively recently, renewable technologies have been better suited to thermally efficient buildings with modest heat demands. It is therefore understandable why it is still common for renewables to be overlooked when specifying or refurbishing heating and domestic hot water (DHW) systems for commercial properties. There is a perception that they will not meet the occupants’ demands, so traditional systems are specified instead.

The truth, however, is that renewable technology used with traditional systems can deliver significant benefits. So-called ‘hybrid solutions’ offer the best of both worlds, complementing each other’s strengths and weaknesses to deliver environmental, fiscal and installation benefits.

Hybrid solutions consist of a boiler (or boiler cascade), a renewable-energy source and two cylinders. This configuration permits a number of installation advantages. Cylinders require volume for stratification to take place, and in commercial properties with a large DHW requirement this can necessitate a very tall cylinder, posing plant room and other logistical challenges. The hybrid system utilises a single-coil pre-heat cylinder, heated via a solar array or heat pump, feeding into the main, second cylinder. Many plant rooms can better accommodate two such cylinders than a single tall cylinder.

A second key benefit of this arrangement is that it is comparatively straightforward to add renewable energy sources to an existing heating system.

Looking to the environmental — and ultimately financial — benefits, a hybrid system permits a significant proportion of heating and DHW demand to be met via natural resources but without the need to have a thermally efficient building capable of being serviced exclusively by renewable sources.

Solar thermal energy can meet a high proportion of the demand for DHW during the summer and be teamed up with boilers to top up as required. This is the Swan Building in Boston, Lincolnshire. It was formerly a feather factory and has been converted into residential accommodation

For example, with solar thermal applications, the solar energy captured via an array can meet a high percentage of DHW demand during the summer months, with the boiler ‘topping up’ as required. During the winter months, when periods of solar gain are fewer, solar thermal can still usefully preheat the water to a higher ambient temperature, requiring less energy from the boiler to then heat.

This benefit of renewable technologies is often overlooked and is illustrative of how the components can work together to fulfil demand and deliver efficiencies. The key to success, however, lies firstly with how the system is specified and secondly the controls strategy put in place to manage the relationship between the two systems.

The overall system should be specified without consideration for any contribution made from the renewable source. This guarantees the system can meet demand during any periods the renewable source is either unable to operate (in the case of solar thermal) or is inefficient to operate (for example, in the case of heat pumps).

A coherent control strategy is vital to ensure optimum efficiency of both the heating system and any renewables, ensuring a seamless changeover from one source to another when increased demand is needed or external conditions dictate it. In this way, a boiler can be called upon to re-heat the cylinder when there is no gain from the renewable source, or boilers can be automatically switched off when all demand can be met.

Renewable technologies are the key to meeting the ever-tightening legislative demands being placed on commercial buildings, and hybrid solutions offer the ideal solution for most commercial properties.

The success of any system, however, lies in its design. This is still an evolving area, and few have the breadth of experience to match that found within the respective manufacturers.

Building owners, facility managers, specifiers, consulting engineers and others would all do well to engage with their chosen manufacturer early in the process to guarantee the best outcome for all.

John Bailey is commercial systems director with Vaillant.

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