Double take

A dual-purpose boiler is claimed to heat homes more efficiently than other CHP systems, and generate electricity at the same time. Berenice Baker reports.

A UK-designed energy appliance that could heat homes while slashing electricity bills is set to undergo field trials with a number of utilities firms.

The HomePowerPlant, developed by Sheffield-based Disenco, is the latest combined heat and power (CHP) system to marry conventional domestic boiler technology with the closed-cycle Stirling engine, and is claimed to produce up to three times the electrical output of competing systems.

The idea is that heat from the boiler is transmitted through a heat exchanger to helium sealed within the engine. Temperature differentials at either end of the engine cause this volume of gas to expand and contract, and the pistons within the engine are driven back and forth. This process generates electricity that can be used immediately or sold back to the National Grid.

On a par with the best combi-boiler systems, the appliance is said to be more than 90 per cent efficient.

The claim that it can also satisfy up to 50 per cent of a typical domestic user’s electricity demands has attracted the attention of utilities giant Centrica, which is testing the technology alongside a fuel cell-based CHP system from Ceres Power. Earlier this month, boiler manufacturer Baxiannounced that Centrica’s British Gas division plans to make a competing Stirling engine based on a CHP system, the Ecogen, available to its customers next year,

Formerly part of Norwegian Stirling engine manufacturer Sigma, which developed a Stirling engine that would fit into micro-CHP, Disenco struck out when Sigma was floated on the Norwegian stock exchange.

Brian Longpré, the company’s investment and corporate manager, bought the rights for this part of the business and moved it to the UK.

Longpré invited the five Norwegian engineers that had been working on it to Sheffield to complete the development of a Stirling engine that would produce 3kW of electricity and incorporate it with a combustion system.

Disenco carried out field trials under government-backed environmental energy body The Carbon Trust, and has since been developing and testing a commercial product.

The company has enlisted the help of three specialist partners. Prodrive in Kenilworth, Warwickshire, tested the original engine and has introduced new manufacturing and engineering techniques to make an engine that is economical and commercially viable. Sentec of Cambridge is working on the system that controls the Stirling engine, the combustion and the G83 directive, by which power is sent to the grid via a user interface. Hull-based Enertek’s part is to commercialise the boiler.

The Stirling engine used in Disenco’s system is a two-piston beta type where heat from the boiler turns linear motion into rotary motion, then produces electricity from a generator, using a rhombic drive.

Alan Dale, Disenco’s chief executive, said the company aims to target domestic and small business markets. ‘Because it generates up to 3kW of electricity and modulates [changes the proportion of electrical versus thermal energy generated] it fits into three to four-bedroom houses and heat-led SMEs, such as a nursing homes, which keep the heating on for much of the day, or a hairdresser’s where there’s a constant demand for hot water,’ he said.

The current model is a direct replacement for a floor-standing boiler and would fit under a worktop in a kitchen, utility room or garage. Stirling engines are quieter than other equivalent engines, with the current model producing 47dB from a metre away — quieter than a normal conversation. Disenco aims to reduce this further as the project progresses.

The company is now concluding tests and will be ready for more field trials next April. By this time the system needs to have received Gas Appliance Directive approval on the combustion side, and conform to the G83 directive, which governs sending power back to the grid.

Disenco has now entered into field trial agreements with international utility companies Centrica, Endessa in Spain and the National Grid in north America.


The production version showing the rhombic drive, base, and the burner

The system’s potential independence from mains gas adds to its overseas appeal. According to Dale, it can also run on propane and a number of other fuels. ‘Once we have a steady revenue stream, our engineers would really like to make versions that run on all sorts of different fuels, including oil and biofuels,’ he said.

‘One of the interesting things that came up recently was a request from Severn Trent water to see whether the system would work on sewer gas — methane. It will, but it needs a bit of development and there’s an associated cost issue. but it’s something we will address.’

Dale said the length of the field trials would be at the discretion of the companies involved, but he believes they will be relatively short, as they follow on the heels of the original Carbon Trust trials.

‘At the same time as the utility companies’ trials we will do our own field trials, and we’re now identifying people who would be suitable to do these field trials,’ he added.

‘Private parties have volunteered a converted barn and a nursing home, and once we’re happy that we’ve got it perfectly right, and perfected the heat balancing, we’ll be ready to go.’

Disenco intends to have HomePowerPlant in production by the third quarter of 2009 and hopes to claim a significant market share of the million boilers replaced in the UK every year. One factor that could tempt potential customers is a bill going through parliament to fix a tariff for electricity sold back to the grid.

‘Currently there is no fixed tariff that the utility companies have to pay for electricity, so each pays what they consider a fair price,’ said Dale. ‘To give you an idea of a fixed tariff, in Germany it’s six times what you pay for it yourself. You can make a lot of money.’

After the trials, Disenco has UK manufacturing partners ready to roll. Autocraft Industries, which is based in Grantham and has three plants in north America and one in the Czech republic, will build the engines. Traditional boiler company, Worcestershire-based Malvern Boilers, will deal with the heating side.

‘Autocraft will make and assemble the engines, and transport them to Malvern Boilers, which will put the boiler, the carcass and the electronics together, carry out a full function test then dispatch to the end user,’ said Dale. ‘They both helped us with the field trial build, so their learning curve is satisfied and they are ready to go straight into mass production.’

Disenco also worked with a number of academic partners on the project, including the Welding Institute and the Casting Institute. The company also carried out some work with Sheffield University, which supported it with funding and helped develop some of the electronics in the early stages of the project.

Once the first HomePowerPlant is in production, Disenco plans to develop a wall-mounted version, then decide whether to develop bigger or smaller versions — down to 1kW or up to 6kW — for different-sized properties. With some further work on the Stirling engine, it could also work as an air conditioning unit.

‘I don’t think we’ll manage to get the whole market share of the million replacement boilers a year, but I’ll settle for 10 per cent,’ said Dale.