Swimming pools have always been expensive facilities to run. Now, in 2023, after more than a decade of crippling austerity, COVID-19 closures and now the energy crisis, swimming pools are facing an unprecedented challenge. Since 2010, England has lost almost 400 pools and Swim England estimates a further 2,000 could disappear by 2030.

In the UK, swimming pool leisure centre assets account for between 10% and 40% of direct carbon emissions from local authorities. In the period 2017–2019, five of Bristol’s main pools – Hengrove, Easton, Henbury, Horfield and Bristol South – consumed a combined average of 13 GWh/y in gas and 4 GWh/y of electricity. If these were gyms or football pitches it is likely the electricity would be the main outlay, but heating water, with its huge specific heat capacity, consumes a lot of energy.

There is no doubt that leisure facilities, and particularly swimming pools, are essential and valuable facilities that are worth the money, but this does not mean the money is there. What is needed is for facilities to be sustainable.

It is perhaps no coincidence that countries that don’t have to pay so much to heat water have more swimming pools and richer swimming cultures. Iceland is a good example and Hungary, whose national sport is water polo, is known for Olympic size pools in even the smallest towns. Visit a pool there and there is a good chance it will also have a yellow-coloured pool containing thermal water, a tell-tale sign the baths use abundant geothermal reserves for heating.

It is perhaps no coincidence that countries that don’t have to pay so much to heat water have more swimming pools and richer swimming cultures.

But, even a seemingly perfect solution is not as simple as it sounds. BVSC Laky Károly Swimming Pool in Zugló, Budapest, is one such example. The pool recently switched to geothermal and now doesn’t pay to heat its water, but drilling is tightly controlled for fear of damaging existing wells. Such pools often have just one shot at drilling, which fortunately paid off in this instance.

And even with the free heat it is still difficult for pools to turn a profit, amplifying just how difficult it is for pools wedded to a fossil fuel subscription that is at the mercy of world events.

In 2021, Exeter in the UK opened one of the world’s most energy efficient leisure centres, a passivhaus standard building. St Sidwells Point Leisure Centre created eight apprenticeships, 15 new entrant jobs and one graduate job, demonstrating the huge opportunity to upskill and provide jobs in the sustainability sector.

Sustainable heating technology

But, aside from knocking down all our leisure centres and rebuilding, a huge energy cost in itself, what can be done to make our pools more sustainable? A technology that has been used for nearly half a century is solar thermal collectors heating water directly. At their most basic, they consist of unglazed black boxes with pipes running through them. In cooler climates, flat plate and evacuated tube designs are used to increase the efficiency and reduce losses.

They excel at heating large volumes of water to low and medium temperatures, provide around three times more energy per m² than solar photovoltaic (PV) systems and have the lowest embodied energy of all solar systems. (This year the International Energy Agency’s Solar Heating and Cooling Programme 71 will begin a programme to build on the data for lifecycle and sustainability of solar heating and cooling systems.)

While solar thermal systems perform at their best in direct irradiation conditions, with efficiencies between 60% and 80% depending on the return temperature of the water, they still generate heat in overcast conditions as PV systems do, although the efficiency is lower at around 30%, still nearly double most PV systems.

Fig 1 shows the total sky direct solar radiation at surface from the European Centre for Medium Range Weather Forecasts (ECMWF), ie clear sky, shortwave irradiation, received by north-western Europe between 2018 and 2022, plotted using the tmap R package.

The ECMWF provides hourly estimates of direct solar radiation reaching the surface based on satellite observations, weather stations and numerical models. While the UK is one of the cloudiest countries in north-western Europe, 1,000 m² of roof space still receives on average 380 MWh/y of direct irradiation.

Fig 1: Accumulated direct radiation, 2018–2022, calculated from ECMWF ERA5 data, plotted using tmap.  
Source: Blaise Kelly

Two European pools  
In 2018, as part of the city of Utrecht’s drive to ensure all roofs have vegetation or solar generation, the swimming pool ‘Zwembad den Hommel’ (Bumblebee Swimming Pool) in the south-west of the city installed 221 evacuated tube panels. The pool was opened in 1962 and has a 25 metre, a 20 metre and a play pool. The system supplies two buffer tanks in the basement and the hottest water supplies the whirlpool which flows into the main pools.

Unfortunately, the system was restricted in size due to the strength of the roof, but this roof space was put to maximum use with all orientations of surface used. Vertical collectors are not uncommon for solar thermal systems as they maximise the low direct sun in winter and reduce overheating periods of the system in summer. However, the variable orientation of the panels created complexities in balancing the system pressures and temperatures.

Utrecht uses solar thermal for heating many of its swimming pools 
Photo: Blaise Kelly

In 2022 the system experienced some problems and heat production dropped off. The system has been flushed for this year and hopefully will be restored. Unfortunately, data on total energy consumption for the site was not available, but it is likely to be a lot more than the annual mean of 48.5 MWh that the solar system provided in this period, highlighting the limitations caused by the inability to fully utilise the roof area. If the pool had been designed from scratch with solar thermal collectors in mind this could have been avoided.

Another example is the Easton Leisure centre in Bristol, which installed a combined solar PV and thermal system in May 2022. It was announced that August that the system had met the entire energy consumption of the centre, although a freedom of information request bizarrely revealed the pool’s 2022 consumption to be higher the previous four years.

Using data from previous operational years for Bristol’s leisure centres, the roof sizes of the buildings and hourly meteorological data from the same period, the total irradiation falling at the sites is theoretically sufficient to supply the energy demand of the sites. But as the examples have shown, achieving that in practice is not always so easy.