Cooling is an aspect of low-temperature engines which is usually neglected, mostly through saying that it is not so important. And that it will be done somehow. That however, sounds to me more like a smoke screen, designed to take attention from an unsolved but important problem.
Why then is it important?
(1) We are working at a comparatively low operating temperature of only 100C, with a condenser (i.e. heat sink) temperature of 40C. The efficiency and the power generation critically depends on the temperature difference, and any small increase in heat sink temperature will have a significant effect on power production.
(2) My initial calculations of cooling water requirement for the 2.5 kW engine showed that 0.3 l/s or 20C water is needed. Does not sound very much? Well, it is 18 l/minute, and 1080 litres of six bath tubs per hour, so the volume mounts up even for small power output. And, wasting drinking water for cooling purposes does not sound like being terribly ecological (even if, as in the UK, it is free).
So, we wold like to get rid of or – to use the technical term – reject the heat generated when the cooling water is heated from 20 to 40C. The only ready substance to do this is air.
The stack-effect cooler
This cooler uses a well known effect, namely that hot air rises. It consists of a cooling loop inside of a chimney which is open at the bottom, as shown in Fig. 1.
The cooler acts as a counter-current heat exchanger, meaning that the air is gradually heated from below so that the temperature of air and water is – theoretically – the same. This allows to cool the water not to the average, but down to the actual temperature of the air. The hot air rises, and the total buoyancy created by the air in the chimney drives the flow and draws cold air from the bottom. The overall effect depends on the temperature difference dT (20K in our case), and the height of the chimney. I estimate that we need 1 m3/s of air, which gives us an 800 mm diameter chimney with a height of 3 m above the coil.
Fig. 1: Stack effect cooler
The system drives itself so to speak. Of course, there are disadvantages:
(1) if the outside temperature goes above 20C, our cooling effect reduces.
(2) We have no active control.
(3) we need a relatively high chimney, which may not be possible everywhere.
The costs of the chimney should not be large, I envisage building it of cardboard which is then painted to protect it against humidity.
The fan-driven cooler
So, option 2 would be a chimney cooler with a fan, Fig. 2. The power requirement of the fan itself is not large, I estimate it as 10-12 Watt to generate an outflow velocity of 2.6 m/s.
You can see that at this power level, heat rejection is solvable with little means. At higher levels it will be more of a problem.
Fig. 2: The fan driven cooler