Low carbon hot water generation

As a nation we have committed to stringent, and legally binding, carbon emission reduction targets through the Climate Change Act 2008. These require an 80% reduction in greenhouse gas emissions by 2050 (against a 1990 benchmark figure) with intermediate targets addressed through regular Carbon Budgets.

As most of the regulated CO2 emission from a building is generated by space heating and hot water production, this represents a fantastic opportunity for the building services sector to play a lead role in driving the UK transition to a low carbon economy.

Until recently, CO2 emission from buildings has been predominantly the focus of the new build sector with emission compliance required through Building Regulations. However, with the introduction of the Carbon Reduction Commitment Energy Efficiency Scheme (CRC), around five thousand large UK organisations operating existing building stock will be pressured to address carbon reduction. Indeed, with fuel prices widely predicted to rise significantly, energy efficiency of all buildings may well fall under scrutiny when operating costs increase as a result.

Another recent addition to the UK policy mix is the Renewable Heat Incentive (RHI), which offers a direct financial incentive to generate heat and hot water using certain low carbon technologies.

Efficient solutions

It is widely accepted that installing separate systems serving space heating and hot water production within a property can deliver the most efficient building services solution. This allows the use of appliances closely matched to the load of each respective service, and negates seasonal inefficiencies typically associated with calorifiers that are heated by main heating plant running at partial load.

Indeed, recent advances in building insulation levels and thermal efficiency driven by Building Regulations Part L have seen hot water overtake space heating as the predominant load in many applications, hence the popularity of direct fired water heaters.

With the best gas-fired water heaters now reaching efficiency levels of up to 109% (net), we are reaching the limit of what this technology type can achieve in isolation. However, this system arrangement lends itself particularly well to the integration of low carbon technologies. Prime examples are the use of heat pumps or solar thermal energy to pre-heat the mains cold feed prior to entering into a water heater.

As shown in Figure 1, the integration of solar thermal technology is a relatively simple concept that is applicable to many buildings, providing sufficient roof space is available on which to mount collector arrays.

When considering the application of solar thermal energy, the design of collector arrays is crucial to performance and longevity of system components.

Glazed flat plate collectors are the most familiar design having been on the market for many years. This collector type must be installed at an inclination of between 20 and 45 degrees to ensure adequate performance. With this in mind they are suited to installation on a pitched roof or, with specific angled mounting frames, to flat roof applications In the latter situation, spacing of collector rows must be considered to avoid inter row shading.

Evacuated tube collectors employ vacuum sealed collector tubes, thereby vastly reducing thermal losses typically associated with the flat plate. This results in a significant efficiency improvement and is generally considered the most efficient method of generating solar hot water, even in wet and windy conditions.  

The collector comprises a number of tubes inserted into a manifold. The solar transfer fluid flows through each tube, hence direct flow.

Collectors are designed to allow the rotation of each tube to meet the desired inclination, therefore it is possible to install the tubes horizontally on a flat roof or even vertically on a building façade. With this in mind, evacuated tubes represent the most flexible and space efficient collector.

Maintenance savings

A recent introduction to the market is the heat pipe collector. Very similar in visual appearance to direct flow evacuated tubes the heat pipe collector uses a dry pocket connection and condenser bulb principle. Each pipe contains a small volume of evaporator fluid and as the pipe warms up the fluid changes to a vapour and rises up the pipe to the condenser bulb. Within the manifold the solar transfer fluid is passed across the dry pocket that houses the condenser. The condenser releases the latent heat of evaporation to the solar transfer fluid and condenses, the condensate returns to the heat pipe and the cycle is repeated. The heat pipes contain a temperature limiting device which operates at 130°C.

In the event of low hot water demand and continued collector heat gain, a bi-metallic valve prevents the evaporated content of the pipe from entering the condenser bulb, thereby protecting the glycol based solar transfer fluid from stagnation damage and eliminating unwanted heat transfer. This design feature makes heat pipes ideally suited to buildings with low summertime hot water demand or irregular demand patterns, such as schools and sports facilities. Owing to this dry pocket design, individual tubes can be replaced without draining down the solar system offering lifetime repair and maintenance savings.

In order to allow the heat pipe evaporation cycle to operate, the collectors must be installed with a minimum inclination of 20 degrees.

With correct application all three types of solar collector can be designed into a system delivering up to 40% of the annual hot water demand of a building from solar energy, which represents a significant reduction in energy cost and CO2 emissions. Given that solar thermal is included in Phase 1 of the RHI it is anticipated that demand for solar thermal installations will increase in both the new build and retrofit markets.

Heat pumps

A similar approach can be used to integrate heat pumps, again providing pre-heat. This is a particularly attractive use of heat pumps as this technology works best at lower supply temperatures. Therefore, a heat pump can be used to raise incoming mains cold water from around 5-10°C up to around 35°C, with a water heater providing the final input to reach the required delivery temperature. By controlling the heat pump to operate at this temperature regime, the efficiency, or co-efficient of performance (COP), is maximised, thereby offering the optimum use of fuel input to the overall system.

Whilst ground source heat pumps are included within Phase 1 of the RHI scheme, unfortunately air source heat pumps are not included from the outset, although this is subject to review in 2012.

Both applications described incorporate a pre-heat store for capturing the respective renewable heat generation, with a direct fired water heater operating to raise the output of this store to the required delivery temperature. 

As this pre-heat store may only reach moderate temperatures, depending upon time of year, with input from the respective renewable heat source, the prevention of Legionella bacteria growth within the hot water system must be addressed.

Scheduled thermal disinfection can be achieved using a timed pump circuit whereby the outlet from the direct fired water heater is circulated through the pre-heat store to raise the store temperature above 60°C for a set period.

Carbon reduction

With the increasing regulatory drive to tackle energy efficiency and carbon reduction, the integration of low carbon technology into heating and hot water schemes will become even more commonplace.

When implementing such technologies it is vitally important to consider the overall building in a holistic manner, paying attention to usage patterns, system loads, operating temperatures and control logic to ensure that the system delivers end user comfort, together with efficient use of fuel and carbon mitigation.

Organisations offering a wide range of technologies, which are able to offer integrated solutions with project application support, will be best placed to deliver carbon reduction and end user satisfaction.