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Energy efficiency in public indoor swimming pools
Energy consumption in indoor swimming pool buildings is very high on account of high indoor air temperatures, increased ventilation heat losses and the energy-intensive water technology. In many towns and municipalities in Germany, numerous swimming pools which had been built in the 1970s are now either in need of extensive refurbishment or have to be demolished completely and rebuilt. There is a backlog of modernisation work and many towns and municipalities are heavily burdened with the high costs of running these old pools. According to [Heiden/Meyrahn 2012], there are 3448 indoor, outdoor and combined swimming pools in Germany alone. There is thus a need to develop energy-optimised solutions and to put these into effect. With this in mind, it was studied how the Passive house concept can be applied as a guiding principle for swimming pool buildings as well; the objective is to achieve optimal thermal comfort (e.g. through high surface temperatures even with large glazing areas) with significantly reduced energy consumption.
The potentials for saving energy in swimming pools are not apparent at first, making the optimisation process difficult. On the basis of steady-state calculations (prototype of a multi-zone PHPP calculation), dynamic simulations, and experiences gained from the planning processes for two pilot projects, various specific options for saving energy were identified by the Passive House Institute. It is intended to elucidate these in consideration of the different interrelationships and dependencies, so that in future projects essential optimisations can be achieved through integrated planning at an early stage.
An introduction to this topic is provided by a systematic investigation of swimming pool buildings [Schulz 2009] which is available free of charge from www.passiv.de (in German). This study was prepared by the Passive House Institute based on the example of the projected new construction of a swimming pool in Lünen (Germany). The key aspects of the planning and construction of the pool, which was opened in September 2011, are documented in a combined report of the different people involved in the work [BGL 2011]. Comprehensive monitoring took place in this project, as well as the second pilot PH swimming pool in Bamberg (Germany). The data analysis from the monitoring is expected to provide fundamental insights into energy flows in such swimming pools and the savings achieved with the Passive House concept.
Basic research based on a demonstration object
The findings relating to energy flows and different ventilation strategies described in this section are partly based on the results of a dynamic simulation of a simplified Passive House swimming pool building with a pool area of around 900 m² (see Figure 1). The amount of water evaporating during operating times is assumed to be 150 kg/h; outside of operating times this is assumed to be 43 kg/h. This moisture must be removed from the building with as little energy input as possible. As shown in Figure 2, the heating demand of the building is around 900 MWh/a. Assuming that the number of visitors is around 600 per day, and if the quantity of fresh water necessary for a good level of hygiene is taken to be 30 litres per person, almost 530 m³ of fresh water are required for the pool each month. Assuming, in addition, that the water loses evaporative energy, the heating demand of the water for the swimming pool is about 800 MWh each year.
|Hall volume |
Treated floor area
Maximum relative humidity: Day
Maximum relative humidity: Night
| 17.800 m³
Layout and boundary conditions of the basic swimming pool model
Energy balance of the model Passive House swimming pool,
assuming dehumidification via ventilation (outdoor air).
Due to the humid and warm indoor climate, energy flows in swimming pool buildings are different from those in buildings with typical indoor conditions (20-25°C / ~50% RH). While transmission heat losses through the building envelope have the greatest effect on the heating energy balance in the “normal” case, ventilation losses take priority in the case of indoor swimming pools. Typically, the indoor air humidity of a pool is dehumidified via ventilation i.e. the humid indoor air is replaced with dry out door air. This leads to continuously high air change rates, determined by the maximum permissible indoor air humidity. The ventilation losses therefore decrease with a higher permissible indoor air humidity. Consistent implementation of the building envelope to the Passive House standard is the key to enabling higher indoor air humidities than is the case with standard building envelopes (according to the German building code (EnEV)), without risking structural damage. For example, double-glazing exhibits an interior surface temperature of 15.7°C at an outdoor temperature of –5°C, which limits the permissible indoor humidity inside the hall to 40%. Passive House suitable triple-glazing with an interior surface temperature of 22.4°C allows just below 60% relative humidity, without a risk of condensation.
Increasing the indoor air humidity requires that all components of the building envelope are of a high quality in terms of thermal efficiency, because the building component with the lowest surface temperature limits the permissible indoor humidity inside the hall.
Unlike buildings used for residential, admistrative or commercial purposes, the indoor temperatures inside swimming pool buildings range between 30 and 35°C. Besides the building physics-related restriction of the air humidity, attention must also be given to the thermal comfort of pool visitors and staff. Factors such as the operative temperature, moisture content of indoor air, clothing and degree of activity, and air speed all affect the assessment of a comfortable indoor climate, which is generally expressed using the PMV index [DIN EN ISO 7730]. A calculation model based on [ASHRAE2005] and [Gagge1986] was prepared in order to apply this index to a wet swimming pool visitor in swimming wear. The indoor air humidity perceived as comfortable decreases in line with rising indoor air temperatures (see Figure 3). Since pool visitors remain in the area around the pool if they are not moving around in the water, their bodies are in radiative heat exchange with the cooler surface of the building envelope. The operative temperature describes the approximate average value derived from the air temperature and the surface temperature of the surrounding building components. The slightly cooler exterior wall also results in a slight drop in the perceived temperature (especially in the case of glazing elements) and thus slightly higher tolerable air humidity inside the hall. If a PMV of +0.51) is set as comfort threshold, this would result in a permissible indoor humidity of 63 to 64% inside the hall.
There are usually different areas with different indoor climate requirements inside a swimming pool building (pool area, changing rooms, entrance hall etc.). If these temperatures differ greatly from each other, thermal decoupling of these different temperature zones by means of insulation of the partition walls and separate ventilation systems is strongly advised, otherwise cross heat flows may lead to undesirable temperatures. This would also increase the transmission and ventilation heat losses significantly. Sufficient planning time should be allowed for the allocation of different zones to different ventilation units, because this creates the fundamental prerequisites for thermal comfort as well as opportunities for saving energy (through regulation of volumetric flows according to demand). When optimising the ventilation units, it is also important to keep in mind the planned setup location inside the building, because the heat recovery efficiency is considerably worsened by long fresh air and exhaust air ducts. Routing and insulation of the supply air ducts must also be taken into account during planning with reference to the surrounding indoor temperatures.
There are many aspects of energy efficiency in swimming pool buildings, whereby the space heating demand only forms a part of the total primary energy balance. In addition there is the heating demand of the water and the electricity requirements of the mechanical systems and pool technology.
Looking at the breakdown of the heating demand into separate areas (Figure 4), it is apparent that the heating demand for the pool water is a determining factor of the model building with the assumed boundary conditions. The largest share of the heat losses from water in the pool is attributable to evaporation enthalpy, even in this optimised case (higher indoor humidity). However, when transferred to the primary energy demand (Figure 5), the ratios change greatly. Although this transfer is based on the project-specific primary energy factors, in most cases it is possible to meet the heating demand with significantly less primary energy expenditure than the electricity demand. Municipalities often own public swimming pools; this opens up possibilities such as the use of combined heat and power plants with waste heat utilisation, which are an interesting option in terms of primary energy. The extent to which it makes sense to use the treated floor area for comparison of the energy efficiency of different pools is still being debated. Often the pool area is used instead.
Air heating demand
Of the different zones of an indoor swimming pool, the hall has the highest heating demand. The highest temperatures (ca. 32°C) are required here and increased air changes are necessary for dehumidification. Particular care must be given to a good quality of the building envelope because the high surface temperatures associated with this permit a relatively high level of humidity inside (up to 64%) without the risk of any structural damage due to condensation. Thus, in comparison with conventionally built indoor swimming pool buildings, the heating demand can be reduced through decreased heat losses from ventilation and transmission, and the power consumption of the ventilation unit can also be reduced significantly (see [Schulz 2009], [BGL 2011], [Peper/Grove-Smith 2013] (all in German)). The performance of the heat exchanger inside the ventilation unit has a decisive influence on the amount of the ventilation heat losses since sensible as well as latent heat can be recovered from areas with high levels of humidity. In addition, exhaust air heat pumps can be used for enthalpy heat recovery. The heat gained via this technology can be used for heating the indoor air or the pool water. A high and reliable SPF (seasonal performance factor) as well as appropriate control of the heat pump is essential. The use of a heat pump should also always be compared with optimised energy generation from an alternative source e.g. through a CHP plant.
As already mentioned, thermal separation of different temperature zones decreases transmission heat losses and, where appropriate, ventilation heat losses.
Within the framework of the basic research on the implementation of the Passive House concept in public swimming pools [Schulz 2009], the influencing factors which limit the maximum indoor humidity were examined for their applicability in highly insulated buildings on the one hand, and the available savings potentials were studied in the context of ventilation strategies on the other hand.
Heating demand of the pool
The heating demand of the water in the pool is affected by the surrounding air humidity since higher humidity levels decrease the rate of evaporation, thus less heating is necessary. Evaporation can also be reduced for example by means improved overflow gutters and water circulation (e.g. by lowering the water levels and circulation underwater thus eliminating evaporation from the gutters during periods when the pool is not being used).
Through the reduction of waste water e.g. by recovering the filtered water, less fresh (cold) water has to be fed in and heated. This can have a significant effect on the heating demand. Different possibilities are available depending on the filter technique being used.
It is advisable to examine on a project-specific basis whether it is feasible to use other types of waste heat (e.g. that emitted by cooling devices for cold pools) or solar heat for meeting the heating demand of the pool water.
Heating demand for hot tap water
The heating demand for hot tap water (showers etc.) depends on the distribution and storage losses as well as the amount of hot water required. Water-saving fixtures, controlled operating times for showers, and a good standard of insulation of the pipes form the basis for optimisation. In particular, long circulation lines and circulation periods with a high temperature due to measures for Legionella prevention lead to high losses.
Today it should be an unchallenged fact to use frequency converters for pumps. As a rule of thumb, the use of highly efficient technology combined with a well thought-out control strategy should be ensured in order to minimise the demand for auxiliary electricity in all areas of a swimming pool.
In the area of ventilation, besides the use of efficient fans and ventilation ductwork with low pressure drops, demand-based control provides vast opportunities for saving energy. The Passive House concept results in lower temperature stratification within the building and lower heating loads, due to which the proportion of re-circulated air in the swimming pool hall can be reduced significantly or even eliminated completely. Air outlets must be planned suitably in order to ensure sufficient air change throughout the room., It should also be possible to maintain a constant pressure drop at the air outlets in spite of the greatly fluctuating air flow volumes. A supply air inlet in the centre of the ceiling and an extract air outlet near the overflow gutters are an example of appropriate and effective air flow in the pool hall.
In other areas of the building such as showers, changing rooms, lobby and staff rooms, room planning with air transfer zones and demand ventilation control based on moisture and/or CO2 are suitable for reducing air flow volumes (by around 50% compared with non-optimised designs). With low pressure losses, these low air flow rates also lead to better fan efficiency in the same duct network and thus a significantly reduced electricity consumption of the ventilation units.
Swimming pool technology
The basic principle for saving energy is to use the shortest possible pipe system with low pressure losses. Besides employing highly energy efficient pumps, selecting a good match of pump specifications for the required application also plays a significant role. Pumps for pool water treatment often have two different operating points, for example for water circulation in the pool and for backwashing of the filter. If this results in the pump operating with a lower degree of efficiency most of the time, then it should be checked whether the use of a separate pumps for the different operation modes could lead to energy savings. Sample water extraction and return, linked to the selection of the measuring cell, also offers a potential for saving electricity. When calculating the energy demand, care should be taken that the efficiency of the pump as well as that of the frequency converter and the motor are taken into account.
The primary and most effective measure for saving energy in relation to lighting is the use of natural light. It should be noted that glazing situated higher up is more effective than glazing at a low level. Windows in the facade are usually inadequate in the case of very deep rooms such as swimming pool halls. Roof lights offer a good opportunity to improve daylight utilisation here. (Domelights of a sufficiently good thermal quality for this purpose are not yet available, unfortunately.). The electricity consumption for artificial lighting should be reduced by means of energy efficient light fixtures, appropriate use of colour, motion sensors, a main switch, and time control etc.
Other electricity consumptions
In order to reduce the electricity consumption of other devices (such as hair dryers, vending machines, elevators, high-pressure cleaners, building management system, CCTV etc.), energy efficient devices should be used and particular attention should be given to control of switch-on times and consumption in standby mode.
Other leisure activities are often offered in larger swimming pool facilities, resulting in additional energy consumption (e.g. kitchens, saunas, solariums, water attractions such as slides etc.). In the case of water attractions, besides the general optimisation of pumps, it is possible to decrease the energy consumption and also the peak loads by means of time-controlled operation (temporary operation, synchrony etc.). Sauna cabins should be sufficiently insulated in accordance with the high difference in temperature compared to the surrounding temperature. The floor must not become too hot. It is possible to heat saunas using gas, which is preferable to heating via electricity in terms of primary energy. The use of separate ventilation units with heat recovery for the sauna cabins is essential from the energy-relevant perspective as well as for reasons of comfort.
Summary and outlook
The energy consumption in indoor swimming pools is notoriously high, but there is also a huge potential for savings. Initial approaches and interrelationships have been discussed in this article, which are intended to assist designers and architects. These findings are based on calculations and experiences gained from basic research work and the planning process for two pilot projects in Germany (Lippe-Bad in Lünen and Bambados in Bamberg). On the basis of the monitoring carried out for these projects, the Passive House Institute will examine the effectiveness of each of the measures implemented for reducing the energy demand after completion of the pools and will evaluate these. See also the article “Monitoring of the Passive House swimming pool in Lünen” (coming soon) for the first monitoring results.
Complex dependencies and interactions between these measures make it very difficult to assess the effectiveness of individual measures and to substantiate these with figures. These measures must be weighed against each other with regard to the savings possible for energy and operation, and a coherent overall concept must be found. The optimum can only be achieved if all specialist planners and contractors involved sit together at one table and if opportunities for active information exchange and discussions are created. This is a fundamental prerequisite which all building owners are expected to meet for a successful project. Investing time to establish a procedure for integrated planning always proves to be worthwhile.
Besides saving energy, the Passive House approach with its highly insulated airtight building envelope provides protection from structural damage - which often occurs in swimming pool buildings - and thus saves costs for maintenance. At this point, it is necessary to draw attention to Energy Performance Contracting for financing - as put into effect in the case of the City of Bremen [Brockmann 2010] for example - particularly for refurbishments of existing indoor swimming pool buildings.
[Schulz 2009] Schulz, Pfluger, Grove-Smith, Kah, Krick: Grundlagenuntersuchung der bauphysikalischen und technischen Bedingungen zur Umsetzung des Passivhauskonzepts im öffentlichen Hallenbad, Darmstadt, Passive House Institute 2009 (only available in German)
[Brockmann 2010] Brockmann, Michael: Energieeinsparcontracting der Bremer Bäder (Seminar Energie- und Ressourceneinsparung, 08.06.10 in Essen)
[Gagge1986] Gagge, A.P., A.P.Fobelets, L.G. Berglund: A standard predictive index of human response to the thermal environment. ASHRAE Transactions, Vol. 92:2B (1986), 709-731.
[VDI 2089]2) Technische Gebäudeausrüstung von Hallenbädern
[ASHRAE2005] American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE): ASHRAE 2005 Fundamentals.
[BGL 2011] Integrale Planung für die Realisierung eines öffentlichen Hallenbades mit Konzepten der Passivhaustechnologie, Bädergesellschaft Lünen, Lünen, 2011 (only available in German)
[Schulz PHT 2009] Schulz, T., The Passive House Standard for indoor swimming pools, 13.th International Passive House Conference, Frankfurt, Passive House Institute 2009
[Gollwitzer PHT 2011] Gollwitzer, E., Grove-Smith, J.: Planning aspects for indoor public pools, 15.th International Passive House Conference, Innsbruck, Passive House Institute 2011
[Peper/Grove-Smith 2013] Peper, S; Grove-Smith, J.: Monitoring Passivhaus-Hallenbad Lippe-Bad Lünen, Darmstadt, Passive House Institute 2013 (only available in German)
Overview of all articles on Passipedia about swimming pools
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