O’Hare (1984) patented a device which passed cooled and heated streams of air through a heat exchanger by changing the pressure of air inside the bellows. The practical usefulness of this device was not shown in detail as in the case of Haneman’s work. Spencer (1989) reported that, in practice, such an engine would produce only a small amount of useful work relative to the collector system size,
and would give little gain compared to the additional maintenance required.