Dominion Energy, owner of the largest power station in New England, US, drafted us in as owner’s engineer on a project to meet stringent new environmental rules. We aided design and construction of two 145m tall towers to cool Brayton Point power plant, the first ever such retrofit. They were delivered 25% faster and for 20% less cost than conventional estimates suggested possible.
Cooling is a key part of the power generation process. There are two water circuits in a power station. One circuit feeds the boilers that produce steam to drive the turbines. Once it has passed through the turbines, the steam must be condensed back into feed water for the boiler. This is the purpose of the second, cooling, circuit.
The 1.6GW Brayton Point plant sits on Mount Hope Bay in southern Massachusetts. Built in the 1960s, its four generating units burn coal, oil and natural gas. It was built with an ‘open loop’ cooling system: water abstracted from adjacent rivers was circulated through the condensers to pick up and remove excess heat, before being discharged into the bay.
To comply with new US Environmental Protection Agency standards governing water quality and biodiversity, Dominion Energy was required to reduce hot water releases. It faced the challenge of either scaling back power generation at Brayton Point to reduce cooling demand – or finding an alternative method of cooling the facility.
Beating the clock by 12 months
Dominion Energy embarked on a fast-track programme. We were appointed in 2008 to design and deliver two new cooling towers, which had to be up and running by May 2012. Industry convention dictated the towers should have been constructed consecutively – but that would have added another year to the project timeframe. For added complexity, the station had to remain operational throughout.
The towers provide ‘closed loop’ cooling. A fixed volume of water is circulated repeatedly, with heat from the power plant released to the atmosphere through evaporation. Hot water from the turbine condensers is pumped to the towers and sprayed from 7600 nozzles. As the droplets of sprayed water fall, evaporative cooling occurs, with heat transferred to surrounding air. This warm damp air is buoyant and rises up the towers, drawing cooler ambient air in at the base of the tower. In this way a perpetual draught is created.
US$100M saving through value engineering
Brayton Point’s conversion from open to closed loop cooling is thought to be the first ever such retrofit. It is also unusual in that only a single pumping stage is required in the circuit. Water from the lower basin is pumped to a height of 41m above sea level. From there gravity takes over, driving circulation through the rest of the cooling system. Reducing reliance on mechanical flow offers greater reliability and reduced operating costs.
We assisted Dominion Energy and contractor Kiewit during construction of the towers, basins and pipework, delivering them for US$550 million – US$100 million below budget. We achieved this through meticulous due diligence and continuously striving to make sure engineering, design and construction were as efficient as possible.
This was aided by a 3D CAD model of the project, which included data on all existing underground structures such as piping and foundations. Prospective bidders were able to use this when writing their tenders. This allowed them to develop more reasonable construction cost estimates.
Our model was then adopted by Kiewit as the basis for detailed design. We reviewed this model every week with the contractor to ensure that the design and engineering were progressing as they should. Over the course of construction, we helped our client save close to US$30 million by identifying value engineering opportunities alone.
By bringing Brayton Point into line with environmental requirements the project has enabled the plant to operate at maximum capacity year-round. Operation of the oil and gas fired Unit 4 had been limited in Summer due to the restrictions on the temperature of water discharged in Mount Hope Bay. With the closed loop system it can now operate at its maximum capacity of 450MW in all seasons – and while surrounding waters remain protected.