I want to design a water heating system for a special type of pool. The system is open-loop, where input water at 10 C has to be heated to 15 C, and then enters the pool and is eventually exhausted out of the pool. The flow rate is about 100 litres/second (6000 litres/minute). Simply heating up the flow requires about 2 MW of power. The temperature of the exhausted output stream doesn't matter, so I was thinking of using a heat exchanger to reclaim some of the heat. A few back-of-the-envelope calculatations led me to believe that because of the large flow rate and the pretty low temperature difference, a very huge and costly heat exchanger would be required to achieve any worthwhile heat reclamation. So I'm now thinking of using a heat pump system to reduce total system size and increase efficiency. The idea is an on-site diesel or gas-powered engine running a compressor, where the input stream is heated by the engine, engine exhaust, and heat pump hot end, and the output stream is cooled by the heat pump cold end. Searching on the internet reveals that systems like this are sometimes used for pools, but the flow rate is typically much lower than what I'm seeking and not scalable. How would I go about designing this?