VSDs in pools – much more than just HVAC

Variable Speed Drives for pool application

It’s not uncommon to hear about people fitting variable speed drives in the buildings that house swimming pools to control HVAC, elevation or solar panels. However, the unique nature of a building containing a 50 metre pool of water that needs to be constantly refreshed, presents a far more interesting challenge for the control engineer. Here, Geoff Renshaw, a director of Powermaster the energy saving technology specialist for pools and spas, explains how a typical pool can reduce its energy bill by up 82% using nothing more than clever engineering.

Before I go on, I should stress that 82% is the record energy saving that Powermaster has provided. It was achieved by combining the effects of variable speed drives, voltage optimisation, heat recovery, remote monitoring and our trademarked IWEMs (Integrated Water and Energy Management) solution in a single application. However, energy savings of 40% are very, very common and it is far from inconceivable that we will repeat or better our 82% record in the medium term future.

The key to making swimming pools more efficient is to fit drives on the motor powering the pool circulation pump itself, not just on the motors in the rest of the building. In many swimming pools the VSD is set manually to decrease the motors speed during the day by 10% and during the evening by 20%. Stepping back to control engineering basics and the ‘power cubed’ law, a speed decrease of 10% produces an average energy saving of about 27.1%. So, during the day, at 90% speed, the load only requires approximately 73% per cent of maximum power and during the evening at 80% it only requires 48%. In essence a relatively small speed change produces a large fall in the energy usage.

As an added bonus, the motor will require less maintenance, because it is no longer running constantly at full speed. This can be a big benefit in a swimming pool, where maintenance often means closure and motors are difficult to reach.

Providing variable speed drives are in place, the use of Powermaster’s IWEMs system can go on to provide even greater energy savings at relatively modest costs. At IWEM’s heart is remote monitoring technology that allows the variable speed drive to control the pump in accordance with the water chemistry of the pool or spa itself. This can be achieved by setting the inverter to control the motor in accordance with the load demand. In a swimming pool, this is calculated in relation to the amount of people in the water, which in turn determines how much chlorine is used. As a result, the chemistry of the water, which can be assessed using a feedback loop from the dosing system, can dictate the behaviour of the inverter. This means that the motor can always run at the slowest required speed, no matter how heavily the pool or spa is being used.

Furthermore, IWEMs can be fine tuned to manage the chemical dosing of the pool or spa, which can in turn lead to further cost reductions – this time in the form of lower expenditure on the chemicals themselves. In effect you are using a simple VSD and some clever stand alone monitoring technology to manage the dosing of the pool.

Furthermore, IWEMs features a completely integrated remote monitoring system that allows the user to view the energy and chemical usage of a swimming pool or spa at the click of a button, via the Internet. This function can also act as a datalogger and an alarming system, sending you an e-mail or text message if there is an urgent event that needs requires attention.

VSDs in pools – explaining the rarity factor
So why do so few companies building and installing pools and spas include variable speed drives as part of their proposal to the building owner? This question is especially pertinent when, in the public sector, a certain degree of energy saving technology is mandatory. Furthermore, why do so few pools and spas make this very small, and often publically funded, capital commitment that will allow them to reduce their expenditure so radically?

My belief is that capital expenditure is prioritised much too highly and operating expenditure is vaguely written off as something that tomorrow’s accountants can deal with.

Let’s look at the example of a variable speed drive controlling the pump or pumps in a swimming pool or spa. The pump is driven by a motor and the entire pump and drive system will cost exponentially more to run during its lifetime than it costs to purchase. A modest addition to the system cost in the form of a variable speed drive, which will control the motor at either pre-set levels or in accordance with the pumps requirements, could reduce the energy bill and thus the lifetime cost of the application by a minimum of 30% in a typical swimming pool or spa application.

As well as finance questions, there is also a cultural and historical issue at play. We have to bear in mind that the swimming pool industry in England is only really 70 years old, having come to fruition in the 1950s. Prior to that, the majority of the pools in the country were the beautiful Victorian structures that campaigners are currently working so hard to save up and down the UK.

The height of technology in older pools was using a massive fan at either end to create water movement and it was felt that the more dynamism in the water the better. The idea that low filtration can actually be the best filtration is relatively new. As a result, using variable speed drives to actually decrease the amount of energy spent on water handling is quite revolutionary in this sector.

Energy saving in Birmingham - two examples in practice
Opened in 1970, Wyndley Leisure Centre is the largest leisure facility of its kind in Birmingham. It boasts a main 25m heated deep water pool for swimming and diving and a separate shallow teaching pool. It also has a 400 metre athletics track, six squash courts, five badminton courts, a gym, an all weather pitch, a children’s playroom and a studio cycling room.

Powermaster’s variable speed drive installation cost only £10,671, including the price of a five year warranty, and saves £3,527 per year, resulting in a payback period of 2.7 years.

Another Birmingham based leisure centre, the Northfield Pool and Fitness Centre has recently undergone a massive refurbishment process. The £3m project included a larger gym, a refitted dance studio and a new sauna and steam suite. Both of the centre’s pools were refitted and the reception area was redesigned. Our part in the project cost just over six thousand pounds and produced a bill reduction of £2,119 per year, resulting in a payback period of just over three years. 

So, next time you read about variable speed drives being installed on a HVAC system in a leisure centre or swimming pool, don’t sit back and chalk up another point for energy saving. In fact, you are reading about a missed opportunity because the motors in the pool itself haven’t been addressed.