Direct-fired water heaters meet renewable energy
Renewable energy can meet a significant proportion of a building’s requirement for hot water — but not all. David Pepper explains how renewable energy should be integrated with direct-fired water heaters.
Low/zero carbon or renewable technologies are now a major consideration for building services design engineers and installation contractors. Among the different types of LZC solutions on the market, two enjoy a high profile. They are solar thermal and air-source heat pumps (ASHP), which are relatively simple to install and can easily be incorporated into a traditional water-heating system.
Both are highly effective LZC solutions. However they will usually only provide 25 to 30% of the annual hot-water consumption of an average commercial property. In all instances, a traditional method of generating hot water will also be needed and will require equal, if not greater, consideration as part of the design process.
In a commercial application, where peak demands often occur, the most effective way to supplement the hot water generated by a solar or ASHP system is to use a direct gas-fired water heater. The LZC equipment will provide pre-heated feed water to the gas-fired water heater. During the winter months pre-heat water temperatures of up to 30°C can often be achieved, with much higher temperatures throughout the summer, providing reduced carbon emissions and fuel consumption.
Climate-change issues continue to dominate the news agenda, and many are now suggesting that LZC/renewable technologies will provide the answer to our energy requirements for the future, including those for heating and hot water. With reference to solar and ASHP hot water systems, the largest proportion of the building’s hot-water demand is to be generated via methods which use fossil fuels, so the selection of this primary source of hot water and its integration with the chosen renewable technology is of great importance.
An air-source heat pump unit is sited externally, and ambient air is used as an energy source to supply pre-heated feed water for a high-efficiency direct gas-fired water heater, providing reductions in fuel consumption, saving both money and carbon emissions.
The outdoor units are fully weatherproofed and generally easy to install. A hot-water circuit between the outdoor unit and the pre-heat cylinder is required, and the user interface controller ensures easy operation and management.
Heat-pump efficiency is often referred to as co-efficient of performance (COP), with up to 4:1 being available on air-source models. Although water temperatures of up to 55°C are achievable, COP is usually at its best when providing water temperatures of around 35°C, and this works extremely well when integrated with high-efficiency condensing gas-fired water heaters.
Direct gas-fired water heaters offer many benefits over an indirect method of providing hot water, such as a heating boiler and hot-water calorifier.
When integrating with renewable technologies, twin-coil cylinders are often selected, with the lower coil being used for pre-heat and the upper coil being the primary heat source. Such methods are often used in domestic applications where space is often restricted, but the overall effect of the renewable gains will be reduced — with the two coils working against each other to an extent. In addition, this indirect method of generating hot water is less efficient than a direct gas-fired water heater system which usually will operate at 10% higher efficiency. The most recent revisions to Part L have recognised this.
Direct gas-fired water heaters are based upon the principles of low storage but with fast recovery of hot water and use fuel more efficiently than an indirect system. Although the concept has been around for more than 30 years, recent developments have included the introduction of products utilising condensing technology, with operating efficiencies reaching as high as 98% based upon gross CV.
In some cases, capital-cost pressure and space restrictions have resulted in indirect hot-water systems with twin-coil cylinders being used in commercial buildings, but in such cases there is the real danger that the benefits from the LZC technology can be cancelled out and the anticipated savings on fuel consumption and carbon emissions may not materialise.
When looking to specify an environmentally conscious water heating solution, it is vital to carefully consider the complete water-heating system and not just the renewable part if UK buildings are to achieve the required reduction in carbon emissions. Commercial applications can require a different approach to domestic properties — particularly those which are subject to heavy peak demand periods.
David Pepper is managing director of Lochinvar.
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