﻿Towards greener hot water
While renewable energy can make a major contribution to producing domestic hot water, it still requires the support of conventional water heating. Yan Evans explores how to get the best from renewable energy.
In the new commercial buildings hot water is fast becoming the dominant thermal load in the property compared to space heating. This is by virtue of improved insulation of the building fabric and increased levels of air tightness. It would therefore seem logical to consider the application of low/zero carbon (LZC) solutions to support the generation of hot water.
Two forms of renewable energy lend themselves to such applications — solar thermal and air source heat pumps.
For commercial buildings, solar thermal systems are becoming regularly specified by architects and building-services engineers on new-build developments as one of the key technologies to reduce fuel consumption and emissions of carbon dioxide. The key driver continues to be the requirements for plannign consent from local authorities. They often stipulate that up to 20% of the property’s energy needs to be satisfied from renewable energy.
These ‘green’ initiatives from local Government are being put in place to support the UK Government’s overall energy strategy and carbon-reduction commitments.
The concept of solar water heating is relatively simple. Roof-mounted solar collectors with high transmission and absorption efficiencies capture energy from incident solar irradiation, passing the heat into a transfer fluid. This heat-transfer fluid is usually a mixture of water and glycol to prevent freezing. The fluid is pumped through a coil in the lower section of an unvented indirect cylinder to heat the stored water. Solar thermal solutions are normally used for heating domestic hot water as when solar irradiation is available during the summer there is little or no need for space heating.
A well designed commercial solar thermal system should be able to satisfy, on average, 30 to 40% of the annual hot-water demand. This value is known as the solar fraction (SF). To achieve this level of energy contribution, the solar-thermal solution needs to be sized on the basis of daily hot-water demand. During the summer months an SF of almost 100% could be achieved, with 15 to 20% during the winter.
However, the presence of the solar-thermal solution should not compromise the sizing and selection of the primary source of hot water. If we consider direct-fired water heaters for generating hot water, the solar thermal system can be used to pre-heat the cold-water inlet.
This solar contribution would reduce the amount of fuel required by the water heater to heat the potable water to the required set point (say 60°C). The degree of pre-heating depends on the generated solar energy which is subject to seasonal variations. And it is for this reason that the sizing of the water heater — which is selected on the basis of the peak hot-water demand — should not be influenced by the solar-thermal system.
Andrews Water Heaters has recently launched a new version of its popular Size-IT program, which now includes a section for the sizing and selection of SOLARflo solar thermal solutions based on a building’s daily hot-water demand. Outputs from this program include the capacity of the solar-thermal indirect cylinder and the collector array — be it glazed flat plates or evacuated-tube solar collectors. This is in addition to sizing and selecting an appropriate water heater(s) to satisfy the peak demand. The latest Size-IT program thus provides support on both water heaters and solar thermal.
Air-source heat pumps
However, solar thermal systems may not always be viable. Insufficient roof space to support enough solar collectors or the absence of a suitable south-facing roof are two reasons why solar thermal may not be selected. In such cases, air-source heat pumps could present a low-carbon solution to support the production of domestic hot water in a similar way to solar-thermal systems.
The CoP of an air source heat pump is typically in the region of 3.0 to 3.5. This CoP is based on an ambient air temperature of 2°C and a load (water) temperature of 35°C. Air-source heat pumps can achieve higher water temperatures of up to 55°C, but their performance would be seriously compromised, with a significant reduction in CoP. This would have a significant effect on the economic and environmental benefits offered by the technology.
The performance of the heat pump is a function of the ambient air temperature (seasonal variations can be expected) and the temperature of the water on the load side of the system. The key in all this is not to expect too much from the heat pump. It is therefore best to control its operation so the outlet water temperature does not exceed, say, 35°C, to maximise the CoP.
Using an air-source heat pump in pre-heat applications would appear to be appropriate to optimise performance. From a sustainability perspective it would be better to raise the water temperature above 35°C by burning gas in the direct-fired water heater than by the heat pump consuming additional electricity at a lower CoP.
With regard to seasonal variations in performance, it follows that we have the same issue with air-source heat pumps as with solar thermal, in that the sizing and selection of the primary appliance generating the hot water should not be influenced by the presence of the LZC technology.
Mixture of technologies
The application of a mixture of conventional products and LZC solutions in the commercial plant is becoming a common theme, driven by ever-increasing pressures to deliver high efficiency and low-carbon heating and hot-water systems. In such configurations it is crucial that the performance of the LZC solution is optimised and at the same time the provision of hot water to the end user is not compromised.
Yan Evans is technical director of Andrews Water Heaters and Potterton Commercial boilers.