One of the most strategic renewable energy components in residential living, and even more so in multi-family buildings is no doubt geothermal hot water (DHW). The reason is simple, everyone needs hot water for domestic purposes, and simple hot water tanks allow the water (and thus the heat) to be stored for later use, with minimal loss, and energy storage is the holy grail for the smart grid. Solar thermal is a superior solution for that reason also, because it is capable of far higher energy density than PV, but also because the storage problem is solved more easily in the form of DHW than it is with PV and batteries.
Unfortunately, manufacturers have a habit of living in silos circumscribed by their respective technologies, all the while pretending that their technologies are the solution to the exclusion of others. This creates the impression that e.g. geothermal DHW and Solar Thermal DHW are competitive solutions, when in fact they are potentially complementary, because of the extremely different behavioral characteristics of the technologies. Particularly, from the standpoint of designing energy generating systems, geothermal energy is base load capacity, i.e. within some limitations it can produce whenever you turn on the switch, whereas Solar and Wind power are peak load generating capacity, which are dependent on the weather, and thus may or may not produce when you turn on the switch. Therefore, solutions that are routinely presented as mutually exclusive, often are complementary instead.
As a result of the technology-centric approach, the field has been plagued by false tries, and in one extreme case a leading manufacturer of geothermal heat pumps, who is promoting their technology for the DHW application, in fact promotes a financial model for the application which leads to inherently wrong systems design. The problem here seems to be that the manufacturers should worry about what happens within their systems, and specifying the proper warranty specs, which become in effect minimum design standards, but NOT design specifications which should be engineered appropriate to the building not to the equipment. Building-centric design is the key. The answers are not the same for all buildings and all markets.
The unfortunate example alluded to here can be found at: Faulty Cost/Benefit Analysis for Earthlinked DHW Systems Designs which is a model apparently intended for design of geothermal DHW systems, but which cannot reliably predict the economic viability of such systems, and moreover makes the design error that generating DHW is the purpose of such systems, when in fact the storage of energy is much more important from the standpoint of designing renewable energy systems. The central problem of dealing with peakloads is how do I store the energy, and thanks to the constant demand for DHW in residential facilities, water storage allows us to harvest economical, renewable peak load power, or even off-peak power from the grid.
In a hybrid system, where the geothermal heatpump preheats the water typically to ca 100F, there is a backup source of heat, to take the water from 100F to the typical storage temperature of 140F (ASHRAE 12), and/or to serve as backup in case of failure. Therefore, preheating the water with geothermal heatpumps only makes sense as long as the cost per delivered BTU of the electricity which drives the heatpumps is lower than the cost per deliverd BTU of the fuel for the backup heating, these days most often Natural Gas. Therefore should the backup fuel be cheaper, it makes no sense to run the heatpump. Particularly when the cost of the two fuels move at different rates, this situation bears watching. Specifically gas is seasonally low in the summer when electricity is seaonally high, and the reverse happens in winter, when electricity is seasonally low, and gas seasonally high.
The model above uses an average for the two input costs, gas and electric in our example, wich is only usable in cases where the prices of the two fuels are far enough apart that the cost per delivered BTU cannot ever cross over, however, when the prices are in a narrow band, and do cross over seasonally, this model will give false indications of the savings that can be obtained by such a system, and if a false positive is used to design a system, the result will be a system which roughly saves money 9 months out of the year, and dis-saves money 3 months out of the year, and if the summer peak is bad enough, it could wipe out the savings of the other nine months. In short, such systems will fail, if they are rigidly based on such evidently false assumptions. Be that as it may, I've seen systems fail for these reasons, and get sold with entirely wrong predictions of their economic value, and in this case the manufacturer's patently faulty financial model is the cause of it.
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