Double take

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.