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Sustainable considerations for swimming pool ventilation
Sustainability
It's no surprise that the indoor climate in swimming pools is an aggressive environment. Swimming pool ventilation is a complex issue and there are many factors that play a role in the technology. Therefore, for some, thinking about sustainability in a swimming pool can seem even more complicated - and no, it's not straightforward.
By Peter Thygesen
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Sustainability can be a vague concept, as the word is used in many different contexts and in recent years has been a buzzword applied to everything from energy to disposable cutlery.
Sustainability is not a word, but a concept that refers to the three pillars of sustainability:
Economic sustainability
Environmental sustainability
Social sustainability
All three are weighted differently depending on the certification scheme, but equally for DGNB, which is the most widely used certification for sustainability in Denmark.
Sustainability is not a word, but a concept that talks about the three type of sustainability.
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Three types of sustainability
Economic and Environmental Sustainability is about limiting resource consumption and environmental impact - not only in the short term during the construction phase, but also in the long term during operation and right through to the recycling of materials. In other words, you should minimise energy consumption and maximise the use of the energy you need to consume.
Social Sustainability is, among other things, about a comfortable and healthy indoor climate for the people who spend time in the swimming pool. In other words, temperature, humidity and the removal of pollutants (THMs). If you want social sustainability, you can't save the energy you add, only the energy you remove from the building.
The energy balance of a swimming pool can be summarised as a general view in Figure 1. A ventilation system in a swimming pool hall extracts approximately 60 % of the energy from the building. This makes the ventilation unit the most energy-consuming technical installation and therefore the most important to optimise.
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The efficiency of a ventilation unit is often assessed on the basis of the heat recovery efficiency in accordance with EN 308 (for dry air). EN 308 applies to dry air. However, few people fail to realise that the air in swimming pools is very humid. Here, the moisture contains more than half of the extracted energy. An appliance with an efficiency of 100 % in accordance with EN 308 can therefore recover less than half of the energy, as this formula shows:
30 ⁰C / 0% RH => enthalpy = 30,0 kJ/kg
30 ⁰C / 54% RH => enthalpy = 66,8 kJ/kg
Therefore, you should combine the passive heat recovery system with an active one in the form of a heat pump, which will be able to recover more of the moisture and thus the energy.
Electricity consumption for air transport
Another efficiency parameter of a ventilation unit is the specific electricity consumption for air transport - also known as the SFP-value.
The electricity consumption of fans depends on how much work they do, i.e. how much pressure they need to provide to move the air in the ventilation system, and how efficient they are at doing so.
A fan cannot convert all the power it uses to move air. Some of the power is converted into heat and is expressed by the efficiency of the fan and motor. You can lower the pressure loss by making the pipework and unit larger.
You need to add energy from another source, as you cannot save the added energy according to the 1st law of thermodynamics:
The 1st law of thermodynamics:
The internal energy of an isolated system is constant. It follows that energy must be added or released to change the internal energy of a system.
This makes sense if you have a heat pump, as every kW saved can be converted into 3 to 7 kW, depending on the efficiency, also known as the COP value for heat pumps. This means that for every kW saved, only 1/3 to 1/7 energy needs to be supplied.
The choice of material is also crucial
It's not just the operation or the indoor climate that influence sustainability. The choice of materials also plays a role when it comes to embodied CO2 and recyclability. Embodied CO2 is a measure of how much energy was used for the production, the transportation and installation of a particular material. It's therefore Important to choosing high-quality materials with a long service life.
On that basis alone, Roskilde Cathedral in Denmark would be the most sustainable building in terms of embedded CO2.
Another factor is the lifespan of the building. It is therefore important to protect a swimming pool with a vacuum to prevent aggressive air from entering the structure and decomposing it.
Not all buildings last 1000 years, so it is important that the materials are recyclable.
A particular challenge with ventilation systems for swimming pools is the heat exchanger, which is large and often contains a lot of embedded CO2. Traditionally, it is made of aluminium, which cannot withstand the chlorinated air. The heat exchanger is often coated, which means that even more energy is needed to recycle the aluminium, as the coating has to be burnt off when it is melted down.
The service life of the building also plays a role.
The swimming pool as a whole
A swimming pool is technically very complex, as many different disciplines work together. Without a holistic understanding, the various technical systems run the risk of working against each other and, in the worst case, destroying each other. This places high demands on interdisciplinarity, especially on management logic. In ventilation technology, for example, there is often a misunderstanding that a damper that is 50 % open means that 50 % air is flowing through.
A swimming pool is technically very complex, as many different disciplines interact.
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A ventilation unit with an integrated heat pump would encompass the disciplines of energy, control, air, water and cooling technology. Energy technology in particular is complex, as it is affected by all disciplines but is not mastered by anyone. To achieve a sustainable system, all these disciplines must work together perfectly.
An example of a system that lacks this holistic understanding is the old 3-fan system shown in Figure 2.
The advantage of this system was supposedly that you could manage with only one fan (V3) for circulating air for heating and thus save energy. Unfortunately, this advantage is a fallacy, as two fans at half load (½Δp) do not consume more energy than one at full load (1Δp). This is because the fan characteristics for EC fans are approximately linear, so that the efficiencies of the fan (ɳv) and motor (ɳm) are the same at half load and full load.
This can be seen from the formula for electrical power consumption.
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This means that no energy is saved during operation, but the additional fan increases construction costs and CO2 emissions. In the endeavour to save energy, a ventilation requirement is also neglected, as no negative pressure can be ensured in the building with just one fan.
The consequence would be to destroy the building and reduce the lifespan of the embedded CO2.
The heat recovery system is only passive, i.e. with a cross or counter flow heat exchanger, and therefore less than half of the energy is recovered.
The system is a customised solution where the control logic is supplied by an automation supplier without a holistic understanding, resulting in an inefficient system at best and destroying the building at worst.
Maximum utilisation with a sustainable ventilation system
So, you get maximum utilisation of the embedded CO2 with a sustainable ventilation system made from durable, recyclable materials.
With a heat recovery system that has a built-in heat pump, you also recover maximum energy, as well as a control logic programmed by a programmer who understands all the disciplines.
For this reason, the ventilation unit should be made for the swimming pool environment, which a customised comfort unit is not.
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Depending on the ratio between CO2 emissions and the price difference between heat and electricity, it could be a ventilation system that has a low pressure in the ductwork and an oversized ventilation unit to achieve a good SFP value.
This article was first published in Svømmebadet and HVAC Magasinet. Danish technical magasines for Swimming Pools -and HVAC-professionals, fall 2023.