Controlling legionella in solar-thermal systems
Published: 06 November, 2008
Preventing legionella in solar-thermal hot water systems needs a thoughtful approach to avoid wasting the potential of the solar energy. Yan Evans explains.
Legionella bacteria are found in water in the natural environment, but it is only in man-made water systems that the bacteria can exist in numbers high enough to cause disease. As the resulting Legionnaires’ Disease can be fatal, there has been a considerable amount of research to define the characteristics of the bacteria.
It is now well established that the bacteria is encouraged to colonise where there is stagnation, accumulation of debris, scale and corrosion, or in lukewarm water between 20 and 45°C.
Research has shown that at 70°C legionella bacteria is killed in seconds, at 60°C over 90% of the bacteria is killed after two minutes, whilst at 50°C it would take two hours to achieve the same level of elimination. As well as temperature, another consideration is the time taken for normal use to completely replace the volume of water held in the storage vessel.
In the UK, health and safety as well as Building Regulations place on suppliers of products and services the duty to design and implement solutions that are effective against the risk of exposure to the legionella bacteria. Manufacturers have developed products and systems for the domestic and commercial markets to meet these demands. This is a continuing process, not least because EU energy policy requires that 20% of energy must be provided by renewables by 2020. This has led to a burgeoning market in the UK for solar water-heating solutions. As the characteristics of a solar thermal heating system may involve lukewarm water remaining at a low turnover for several days in the storage vessel, this might suggest a greater risk of legionella bacteria contamination.
A closer look at the design considerations applicable to commercial solar water-heating applications will show that this not the case if the correct design features and operational procedures are employed.
The key figures, already identified, are the temperatures between which legionella bacteria can develop. There will be periods during the year when there will be insufficient solar irradiation to heat the cold water in the solar cylinder to the required outlet temperature of, say, 60°C. In the UK during the winter there is only sufficient available irradiation for solar thermal systems to satisfy around 20% of the hot-water demand, the balance being provided by gas-fired boilers or water heaters.
This lower solar energy contribution can still reduce the fuel consumption of the primary heating appliance by preheating the incoming cold water from, say, 10°C to, perhaps 20°C. However, there is still the risk of lukewarm water residing for periods of time in the solar cylinder and the associated risk of legionella bacteria developing.
In summer months there should be sufficient solar energy to meet all the demand without support from the fossil fuelled appliances. During these periods the risk of legionella developing is greatly reduced.
However, throughout the year there will be a need to pasteurise the solar cylinder at the correct temperature to eliminate bacterial contamination.
The operational philosophy for pasteurising the solar cylinder depends whether it is a twin-coil design with one coil served by a boiler or a single-coil cylinder used with a direct-fired water heater. In both cases the primary heating appliance provides the heat necessary for disinfecting the cylinder.
The specific requirements for mitigating the risk of the development of legionella bacteria are detailed in the HSE Code of Practice and Guidance L8, ‘The control of legionella bacteria in water systems’. However, it is not categorically clear in this document how frequently the anti-legionella cycle has to be operated. We would suggest that pasteurising at least once a week for a period for at least an hour to ensure disinfection. It is, however, the decision of the end user what anti-legionella strategy should be adopted, depending on the risk of the development of the bacteria and the property/application in question.
The frequency of the cycle and the time of day pasteurisation occurs have a significant effect on the performance and operation of the solar thermal system.
The solar hot-water cylinder and the accompanying collector array should be sized for the daily hot-water demand. Heating the cylinder water up to 60°C every day could negate any solar contribution. By taking the whole content of the cylinder up to that temperature the load offered to the solar collector array is effectively removed.
Equally, if the anti-legionella cycle is operated at say midnight on a Sunday to increase the water temperature throughout the cylinder to 60°C and there is no hot-water demand during the early hours of the Monday morning, e.g. in an office block or school, then when the Sun rises there is no hot-water load to gain the benefit of the available solar irradiation.
In twin-coil solar cylinders the lower coil is heated by the solar collector array and the top coil by the primary heating appliance. A temperature sensor is required in the top segment of the solar cylinder to measure the water temperature. If the temperature is sufficient to perform pasteurisation (60°C), the solar control unit will initiate the anti-legionella cycle and run the cylinder de-stratification pump. The latter moves the hot water from the top segment of the solar cylinder into the lower segment. This
hot water rises through the cylinder eliminating any legionella bacteria that may be present. If the water in the top segment of the cylinder is not at a sufficient temperature to perform pasteurisation, then the solar control unit would initiate operation of the boiler serving the top coil to provide energy to heat the water to the required temperature prior to de-stratification.
If the demand profile of the building is well known, the anti-legionella cycle could be operated when the peak hot-water demand occurs, when the boiler serving the top coil is very likely to be operating anyway. Pasteurising the cylinder during this period would prevent the need to operate the boiler and consume fossil fuel simply to kill bacteria, which would partially negate the carbon reduction benefit of the solar thermal system.
Single-coil solar cylinders can be used to pre-heat the cold water inlet to a direct-fired water heater, reducing the appliance fuel consumption required to heat the water to the desired outlet temperature. However, in single coil cylinders there is no primary source of heat with which to perform pasteurisation. The use of immersion heaters to carry out this function should be frowned upon, as the carbon intensity of the electricity would partially negate the environmental benefit of the solar thermal installation.
Direct-fired storage water heaters are designed to ensure that the water stored is always maintained at 60°C or above. This water stored within the heater can be used to pasteurise the solar pre-heat cylinder. A shunt pump should be installed between the water and the solar storage cylinder. Hot water is pumped between the hot-water outlet of the water heater and an inlet port as low as possible on the solar cylinder. This operation can be controlled by the solar control unit. Hot water then rises through the cylinder to eliminate any legionella bacteria that may be present.
To meet their obligation to provide effective protection against the risk of exposure to the legionella bacteria, owners and managers of commercial buildings should look for clear principles in the design of solar thermal water-heating systems.
When considering the application of solar thermal systems, end users and specifiers should consider the highly important questions as to whether the equipment under consideration been designed to ensure pasteurisation and disinfection of legionella bacteria and whether the strategy for operating the anti-legionella cycle ensures that the most energy efficient method is employed without negating the carbon-reduction benefit of the solar thermal installation.
Yan Evans is technical director of Andrews Water Heaters
and Potterton Commercial.