How Bristol’s new district heating network turns harbor water into a hot shower

It looks set to revolutionize the way people living in Bristol – particularly if you live or work in one of the many new buildings popping up from Old Market to Redcliffe or Bedminster to Temple Meads – stay warm and have hot water. But it sounds like something out of science fiction, and the reality could be a plot from Doctor Who.

Because a team of engineers are now pumping the cold, murky water out of the floating harbor near Castle Park and somehow using it to send clean, warm water to radiators, showers and kitchens in inner-city homes.

But how exactly does it work? We took a tour of the heating network center in Vattenfall Castle Park and not only tried to find out, but also tried to understand when they explained it to us.

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Martin Rouse, Operations Manager at Castle Park Heat Network Centre, started with the basics. He pointed down into the water of the Floating Harbour, as the old course of the River Avon curved around Finzels Reach to Bristol Bridge and beyond. “We extract 517 cubic meters of water and send it here,” he explained, turning to the buildings behind him, “and we take four degrees of heat out of that water.

“So let’s say the water we’re taking out is 15C. We will take four degrees out and put the same water back into the Floating Harbor at 11 C,” he added. “There is so much water in the Floating Harbor that it makes no difference to the overall harbor. Even if you had several heat pumps along the floating harbor doing the same thing, it would take an absolute age to affect even 1% of the amount of temperature in the harbor itself.”

The floating harbor is a semi-sealed body of water. It is blocked by the Cumberland Basin lock gates, which let some out regularly, and there is a constant small flow of water from the River Frome, which passes under the town centre. Everything is heavily regulated, but it’s perfect for heat pump technology – it’s big enough, it’s not big and unpredictable, and it stays pretty much the same temperature.

When the water is sucked into the buildings near Castle Park is where the complicated magic of science happens. It is first filtered a little – large pieces of mud are taken out, but put back in when returned at the end of the process.

Then the water is fed through a pipe. That pipe is sprayed with cold ammonia, and naturally the water inside the pipe gets a little colder — four degrees is the goal, Martin explained.

Once this has happened, the water has done its job and is then returned back to the Floating Port, slightly cooler than it was before. This is the last involvement of the water in the Floating Port – now it won’t end up in your bathroom.

All it was used for was its heat, which feels counterintuitive considering it’s not exactly warm to begin with. Cooling the water by spraying it with cold ammonia means that those four degrees have to go somewhere – those are the laws of physics. These four degrees of heat have actually been transferred into the cold ammonia spray, which is now slightly less freezing than before.

The warmer-than-normal ammonia is then compressed in a series of high-pressure pipes and chambers and then evaporates. The high-pressure oil is then heated in other pipes. The four degrees of heat from the harbor water are transferred a second time to the oil through a heat exchanger, which – much like a car radiator works, or – perhaps more pertinently, a freezer works, but conversely, the oil in closed pipes end up getting hotter and hotter, just like the back of a freezer is hot.

That oil is then used to heat water – lots of it. It didn’t come from the harbor itself, just the regular water source, but it’s dyed bright green, filtered and purified, and will stay in a giant closed system forever. The hot oil heats it up to 75°C and is stored in a huge brown storage unit, much like an immersion tank in an ordinary house, right next to the two factory buildings in Castle Park.

From there, it is sent around a series of huge underground pipes into the first district heating network, which serves the Castle Park and the Old Square.

The first port of call is the new Castle Park View tower block – 375 flats in five different blocks, while the furthest radiator system is Hannah More Primary School in the back streets off Temple Way.

But the bright green hot water doesn’t actually enter the buildings, it’s just a taxi driver carrying the precious cargo – heat. The 75C temperature water is then sent through pipes at the entrance of each building which hits the building’s own hot water systems through another series of heat exchangers and again the heat is transferred a third time.

The bright green water from Castle Park – it’s only green with a harmless dye so it’s easy to see if any leaks – then it goes back, hopefully cooler again, to Castle Park where it’s back in the storage tank ready to be heated again.

The heat, after leaving the green water in the large pipes under the road, is transferred to the closed water systems of any building that has been connected to the District Heating Network, but even then, it is still not the water that people will have in showers or in their kitchens. That now, the freshly warm water circulates around the building, heating the radiators, but also heating the boilers in the apartments which take cold water from the mains and heat it – thus transferring the heat a fourth time.

It is, Martin explained, quite complicated, but also very effective. Obviously, at Castle Park, there is some serious micro-science going on, and it needs energy to start with. For every unit of energy (in the form of power to pressurize ammonia and oil) they use, they receive the equivalent of three times as much heat in the form of a storage unit filled with 90,000 liters of very hot water.

The entire system – apart from the bit at the beginning where the port water is drawn in, cooled and put back – is completely closed, including the ammonia. “Ammonia is a natural compound that has been used in refrigeration systems for over 100 years. Ammonia used in industrial-scale water source heat pumps is relatively new, with the Castle Park water source heat pump being only the third example in the UK,” said a Vattenfall spokesperson.

Three different networks are currently being developed and expanded in Bristol. The former, which serves Castle Park in the Old Market, is the only one to use water from the Floating Harbour.

The others are at Redcliffe, which is also operational, and Bedminster, which is still being built – as drivers on the A38 Malago Road know all too well, which has been closed for two years northbound while the pipes are laid.

Ultimately, the plan is to join the three networks together so that they are all vented by harbor water heat and other smaller booster heat producers along the way. And then add more. When the developments in the Temple District see conference centers and university halls and thousands of homes, they will be connected. When the Frome Gateway regeneration project starts, it will have its own district heating network to join. As the transformation of the town center begins – with thousands of new homes, student accommodation, offices and shops in what is now Debenhams, The Galleries, Premier Inn and other Broadmead buildings – a heat network can be part of that regeneration.

The idea is a big one. The more buildings take in the heat, the more efficiently it will work – the system relies on that bright green hot water moving around the closed loop and cooling down.

“It’s like the radiator in your car,” Martin explained. “Your engine heats up, it puts heat into the water to cool down. It’s heat exchanger and it’s mechanical, it doesn’t use any power.

“It’s pumped from the pump in the engine as it goes along the same way we pump it here into the network.

“We don’t get all the heat from there for free,” he explained, pointing again to the water in the Floating Harbor. “We add one kilowatt of electricity and take out three kilowatts of heat. We get heat from there, but we use less energy to heat people’s properties and get all their hot water than we would by using gas or using any form of electric heating,” he added.