The Geothermal Water Heater (GWHP) US Patent #7,827,814 http://www.halslater.com/US7827814.pdf
What it is: The GWHP is a new, highly efficient water heating design for single or multi-family residences in temperate and tropical climates.
How it works: The GWHP extracts excess heat from the cold water used throughout the home with a water-to-water heat pump to heat the hot water used throughout the home. The key factors are that: 1) the typical residence uses three to four times as much cold water as hot water, 2) in some climates the cold water is about 15º-20ºF warmer than it needs to be and 3) water-to-water heat pumps are, on average, more than twice as efficient as air-to-water heat pumps.
How it compares to the current leader in water heater efficiency, the Air-to-Water Heat Pump (AWHP): The second-generation prototype AWHP operated at an EF of 1.92 compared to our first generation prototype unit which operated at 2.14. The third generation AWHP has an average ODE of 1.27 in the field, which is about the same as the first generation GWHP of 1.28. Fully optimized per the final report’s Recommendations, the GWHP should be able to reach an EF over 3.25 and an ODE above 2.5.
How the total installed cost compares: The cost to manufacture the GWHP should be under $200 higher than that of the AWHP. Installation is slightly more complicated since the system must supply cold water to the home as well as the hot but no difficulties were encountered in the three system installed for the tests. These added costs should be more than offset by the fact that, as a geothermal appliance, the unit qualifies for a 30% personal tax credit. If this design is powered by solar electric panels it can lower the cost of solar hot water by 20% compared to solar thermal and provide a net 100% solar hot water fraction with no maintenance or risk of freezing, boiling or scaling.