The present invention relates generally to a free-piston Stirling engine, and particularly to improvements in resistive load control of such an engine.

BACKGROUND OF THE INVENTION

A free-piston Stirling engine (FPSE) is a closed-cycle, reversible heat engine which converts heat into work by moving a confined volume of working gas between a relatively warmer heat acceptor and a relatively cooler heat rejecter. The resulting alternating, cyclical, expansion and compression of the internal working gas provides an oscillating pressure wave that drives a piston to oscillate substantially sinusoidally in linear reciprocation. The piston is mechanically linked to a ring of permanent magnets that it drives in reciprocation within the winding or coil of the linear alternator thereby inducing a voltage across the winding terminals.

An FPSE is usually controlled by a high power resistive load during transient modes, such as start, stop, or "no-external load" condition. The resistive load may also be used to modulate the system when a different operating power point is required.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved resistive load control of free- piston Stirling engines, as is explained more in detail hereinbelow.

In accordance with an embodiment of the present invention, a high power resistive load of an FPSE is integrated into a water loop of a combined heat and power (CHP) appliance (system). In a typical CHP appliance, such as a micro combined heat and power (mCHP) appliance, heat (usually in the form of hot air or water) and electricity are the two forms of energy that are generated. In such a system, the heat produced from a combustion process can drive an electric generator, as well as heat up water, which means the CHP appliance generally already has a water loop system. The invention takes advantage of the water system that already exists in the CHP system to cool down the high power load resistor. The heat generated by the resistor is not wasted; rather it is recuperated into the water heating system which offsets losses and leads to higher overall efficiency.

There is thus provided in accordance with a non-limiting embodiment of the invention a system that includes a combined heat and power system that includes a water loop. A free-piston Stirling engine includes a heat exchanger that outputs to a resistor load, and the resistor load is water-cooled by the water loop.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

Fig. 1 is a simplified illustration of a free-piston Stirling engine control system, constructed and operative in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to Fig. 1, which illustrates a free-piston Stirling engine control system (combined heat and power system) 10, constructed and operative in accordance with a non-limiting embodiment of the present invention.

System 10 includes a FPSE 12, which may include a cold water heat exchanger 14 with a cold water input 16. FPSE 12 is integrated with a hot water boiler or buffer tank 20 of a combined heat and power appliance 21, such as a mCHP (micro combined heat and power) appliance, which has an existing water loop, designated generally as water loop 22. In one embodiment, a control water-cooled resistor load 18 is added to loop 22, so that the output of cold water heat exchanger 14 flows to water-cooled resistor load 18, and from there, to hot water boiler or buffer tank 20. Additionally or alternatively, a heater resistor load 24 may be at least partially immersed in hot water boiler or buffer tank 20.

The high power load resistor may be manufactured by a length of resistive wire coiled around the water cooling loop piping and attached to the pipe in such a way that prevents electrical shorts while still maintaining good thermal contact between the wire and the pipe.

Integrating the high power resistor 18 and/or 24 into the water cooling loop 22 has several benefits, such as:

(1) The dissipative heat generated by the control resistor 18/24 is directly recuperated back into the water loop 22 and not wasted, thus improving the system level efficiency.

(2) Water cooled resistors are quite compact and do not require elaborate heat sinks to prevent them from burning out.

(3) Water cooled power resistors or immersed heaters generally lead to an inexpensive resistive load solution when compared to comparable air cooled resistor solutions.