Wobble yoke and Stirling engine are key to reliable, green CHP system

A domestic combined heat and power system based on a Stirling engine design and wobble yoke may also find applications in industrial applications. Dean Palmer reports

A domestic combined heat and power system based on a Stirling engine design and wobble yoke may also find applications in industrial applications. Dean Palmer reports

A ground breaking micro combined heat and power (CHP) system has been developed that uses a novel, four-cylinder Stirling engine and 'wobble yoke' design, which keeps side forces to a minimum and which results in a low noise, low carbon dioxide emissions, high overall efficiency design.

The system, developed by New Zealand based firm Whisper Tech, has already found a suitable market in the UK. The 'WhisperGen' system emits 20% less carbon dioxide than conventional domestic boiler systems which equates to around one to two tonnes of carbon dioxide per home per year. It is little surprise then that energy provider Powergen has just signed an exclusive agreement with Whisper Tech to supply of 80,000 'WhisperGen' micro-CHP units over the next five years for installing into UK homes.

The AC version of WhisperGen operates as a floor-mounted, fully automatic boiler which runs on natural gas and provides up to 8kW of thermal energy for hot water and space heating, while also generating 1.2kW AC power output at 230V.

The system is around the size of domestic dishwasher unit but the mechanics inside the unit are completely unique. The core of the system works on a Stirling engine design, which incorporates four pistons and a 'wobble yoke'. The unit is also fairly quiet with noise levels of 45 47dBA at one metre.

The Stirling engine operates on the principle that heated gas expands and cooled gas then contracts. This expansion and contraction within the cylinder forces a piston to move in and out. Basically, gas is heated in one place and then transferred to another chamber. The pistons work at around 1,600 to 2,000 cycles per minute.

David Moriarty, UK director of Whisper Tech, demonstrated the system to Eureka. He explained: "One mass of gas in the cylinder is cycled back and forth by piston motion. There is no valve gear and no air or fuel is taken into the cylinder from the atmosphere. This allows the engine to operate in a very clean and quiet manner.

"Heat needs to be supplied to the hot heat exchanger by a simple, continuous external flame. Cooling water removes heat from the cold heat exchanger and heat gained by this water can be used for domestic hot water and central heating," he added.

To produce a small engine with very low vibration, smooth torque and simple construction, WhisperGen combines four piston-cylinder sets in a tandem arrangement. The hot end of one cylinder is attached to the cold end of the adjacent cylinder.

The key to the next part of the system is a unique world-wide patent pending wobble yoke mechanism which converts linear motion of the four pistons into rotary motion required to drive the alternator. As Moriarty explained: "A unique wobble yoke connects the pistons to a rotating shaft and alternator, which are sealed into the compact housing. The pistons have a series of PTFE seals and guides to retain the gas within the cylinders. Unlike an internal combustion engine in a car, no oil lubrication is required in the cylinders, which means that the piston seals and guides operate in a dry environment."

There are two beams and two yokes, both made from cast iron. There are 16 bearings (which oscillate rather then rotate) used in the design a mixture of taper and automotive standard needle roller bearings.

All the moving parts and the alternator are housed within a pressurised nitrogen casing. It's a fully enclosed, sealed-for-life housing with only two wires protruding from it to connect the unit to the power supply. Nitrogen is the normal fuel used even though this is not as efficient as helium or hydrogen. However, with hydrogen or helium, the smaller sized gas particles are very difficult to seal so, as Moriarty explained, "a compromise was made on the overall system efficiency."

And efficiencies are pretty impressive. Total system efficiency is between 94% and 96% based on the lower calorific value of the fuel, with electrical efficiency at 12%. Moriarty explained that the key to bringing WhisperGen to market was that compromises had to be made so that the unit could be manufactured at reasonable cost. "There is little point in developing a super-efficient Stirling engine and wobble yoke if you can't manufacture it easily or cheaply," he continued.

He went on to explain that, while seal and guide life has normally been a limiting factor in Stirling engines, the wobble yoke development enables power to be transferred from the pistons to the alternator in a way that minimises the side load on the piston seals and guides. When questioned about the actual life of the piston seals and guides, Moriarty said that the seals had withstood in excess of 30,000 hours (or 12 to 15 years' domestic operation) in accelerated life tests.

A built-in electronic control unit (ECU) manages all aspects of the system and uses an LCD control panel to display real time operating conditions.

The burner consumes air and fuel, the rate of which is optimised by a separate automatic burner control unit (microcomputer) integrated within the ECU. This maximises combustion efficiency and minimises carbon dioxide exhaust emissions by managing gas flow and ignition.

A passive safety circuit is built into the engine and includes a series of switches independent of the ECU. If any switch in the circuit is tripped, the burner control shuts down and the WhisperGen stops operating. The ECU also monitors the electrical output and central heating system operation.

In the case of a mains failure, disconnect or deviation from prescribed limits, the system disconnects from the mains and shuts down. Once the Grid has returned to normal, the system will reconnect automatically.

The heat management system monitors the coolant entering and leaving WhisperGen. The room temperature, hot water cylinder temperature and the outside air temperature are also closely monitored. This means the controller can infer the heat requirements of the house and adjust the power output accordingly for maximum comfort and economical benefits to the user.

Moriarty told Eureka: "We're developing sophisticated algorithms that take account of outside air temperature, room temperature and other factors, start and stop times of the engine, coolant targets and other operating settings can be adjusted on the fly to further enhance the WhisperGen's characteristics and performance."

He also said he expects domestic users to save around £150 to £200 a year on their fuel bills as a result of installing WhisperGen. The price of WhisperGen is currently set at around £3,000, but this should fall as production increases. And Powergen will probably offer customers a lease type payment plan which will help tie customers in to a two or three-year deal, preventing customers from 'swapping' energy providers every year or so.

The real benefits are to Powergen though. As well as 5% on VAT, the company should also benefit from Climate Change Levy tax incentives through WhisperGen's reduced carbon dioxide emissions.

The curious thing about this unique design is that Whisper Tech and its technical director Don Clucas, developed the Stirling engine and wobble yoke design first, then looked for suitable applications, not generally considered the best way of bringing a product to market.
But the applications for WhisperGen look encouraging. As well as the domestic CHP application, Moriarty expects a host of applications to arise for the company's DC alternator version. The marine industry already uses more than 500 DC type WhisperGen units, as onboard CHP systems, and a remote station in the Antarctic also uses two units.

Moriarty also mentioned theme parks, radio cell phone towers, national parks, chillers for air conditioning systems and turbines as other areas the company was looking at.

Pointers

* WhisperGen is a micro combined heat and power system aimed primarily at domestic hot water and heating markets

* At the core of the design is a unique four-cylinder Stirling engine and wobble yoke which minimise side loads and result in a maintenance-free, lube-free system

* Sophisticated controls and electronics use complex algorithms to monitor and control overall system efficiency and keep carbon dioxide emissions to a minimum

Author
Tom Shelley

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