Cool ways of using low grade Heat Sources from Cooling and Surplus Heat for heating of Energy Efficient Buildings with new Low Temperature District Heating (LTDH) Solutions. Report on solutions for avoiding risk of legionella
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Legionellae are bacteria that naturally exist in freshwater environments, but have also been found in seawater and soils. Legionella can cause illnesses such as Legionnaires' disease or Pontiac Fever where the Legionnaires' disease is a severe variant of Pneumonia and Pontiac Fever is a milder influenza like variant. Legionella bacteria can grow in domestic hot water systems (DHW systems) and in order to prohibit bacterial growth, national temperature levels on DHW systems have been set. As the district heating industry strives to lower the supply temperature in district heating systems to achieve higher system efficiency, new solutions might be needed for solving the problems of Legionella growth in the customers’ domestic hot water systems. The aim of this report is to: • give an overview over the regulations in domestic hot water systems in terms of Legionella safety in six European countries (Sweden, Denmark, Norway, Finland, Germany and France) • give an overview on different techniques for Legionella prevention. • make an analysis on the advantages and disadvantages of the different techniques and how these techniques comply with today’s regulation of domestic hot water systems as well as to analyse the applicability of the different methods when used within a low temperature district heating context. • make a statistical outlook on the incidence of Legionnaires disease in the six selected countries and how the incidence levels relate to regulated temperature levels for domestic hot water systems. The review of national rules regarding Legionella safety in DHW systems shows that the legislation in all countries is based on maintaining a sufficient temperature in domestic hot water systems in order to make an unfavourable environment to the Legionella bacteria. There are no rules that deal with the control of bacterial levels of Legionella in the water. The temperature level requirements in the six different countries differ. For most countries, the DHW temperature requirements are set to minimum 50 °C at the drain point and 60 °C in storage tanks with stagnant water. Some countries have chosen to set the temperature requirements even higher, for example Norway that declares that Legionella should be controlled by keeping the temperature in circulating water systems above 65 °C. Other countries, such as France and Germany, use special legislations for small DHW systems (this special legislation is often referred to as ‘the 3-litre-rule’), and in Denmark a DHW temperature of 45 °C is permitted at times of peak flow. Our overview over feasible technical solutions for preventing Legionella growth in domestic hot water systems shows that there are several alternative techniques that theoretically could prevent Legionella growth besides the conventional one based on keeping a high water temperature. Many of these methods, however, come to short because they cannot be viewed as completely safe or as good long-term solutions, or they have not been commercialised and fully tested yet. Because of the legal temperature requirements in domestic hot water systems, the alternative techniques for Legionella control could not be used as single methods if ultra-low temperature district heating is used (with a supply temperature of 50 °C), since the legal temperature requirements would then not be met. That means that today's legislation does not support ultra-low supply temperature in district heating grids in most EU countries unless the DHW temperature in the customer installation can be boosted using local heating solutions, such as electric heat tracing, micro heat pumps or instantaneous water heaters. In countries applying the 3litre-rule, DHW preparation in decentralized DH substations could be a solution. When decentralized DH substations is used, the DHW is heated up instantly in a small heat exchanger with short residence time (seconds). The question arises if such heat-up has a significant influence on Legionella survival dependant on if the set point temperature is 55 or 45 °C. More research is needed to bring clarity if the small DHW systems can be assumed to be Legionella safe. If the answer is yes, this could open up for usage of ULTDH for direct preparation of DHW at 45 C° in instantaneous heaters. According to statistics from the European Working Group for Legionella Infections (EWGLI), the countries with the lower temperature requirements actually do show more incidences of Legionnaires’ disease. The findings are interesting although it is not possible to establish causal relationship by these variables within the framework of this study - if it is even possible to determine this at all. There may be other, underlying, variables that can explain this relationship. Nevertheless, for health reasons, and for PR reasons, it may be wise to use a precautionary principle for alternative techniques to prevent Legionella growth. It would not be fruitful to build a heating system that increases residents' risk of getting Legionella; furthermore, such a development could conduce to denigrate district heating reputation. If the research on Legionella safety in small DHW systems can show good results, there will be reason to argue for some legal changes. Today’s legal requirements in DHW systems can then be seen to unnecessarily aggravate possible system efficiency in the district heating business that comes with lower supply temperatures.
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Ämnesklassifikation (UKÄ) – OBLIGATORISK
|Status||Published - 2018|