There's a lot of energy saving innovations that supposedly all pay for themselves eventually.

For example, suppose you're trying to make the ultimate hot water system.

  So you install a solar hot water heater on your roof.  Presto, 3-10 year payback, free fuel.  Then you add a drain heat recovery system that preheats the water going in to the holding tank for the solar hot water system. After the holding tank, THEN you have an ON DEMAND gas hot water heater with electronic flame control to heat the water when you need it.  Oh, and you throw in a desuperheater as WELL that preheats the cold water coming in, with a relay controlled water valve that bypasses the drain heat recovery system when the heat pump is on and in cooling mode.

The problem is that each one of these systems alone would substantially reduce your costs from baseline.   Let's suppose that each system cuts your energy usage in half.  You'd get diminishing returns with each system you add, since there are fixed costs with each of the technologies listed that don't go down with smaller capacity systems.  You can only use so much hot water in a day.

So, I'm guessing that the solution that saves the most money is to pick the single most cost effective of all these technologies and to install only that.  What do you think?  How would you analyze the economics for a complex scenario where you have purchased multiple systems?

Ever heard the phrase 'Too much of a good thing'?
It is easy enough to overdo something, even energy efficiency.
With the variety of options available, one must choose the one option (or combination) that seems best suited to the circumstances at hand.
I, personally, start with the simplest and work up from there.

Good question.  I'd be interested to hear the answers.  I can add my particular situation.  I recently decided to go with geo-exchange and a desuperheater for our new home.  I had been very interested in solar thermal as well for DHW.  Now, I'm wondering whether adding solar thermal would result in wasted heating.  I'm not overly concerned with the payback of these systems, just whether they're overkill for the amount of DHW we need.

The house will normally have 6 adults which should result in a fair amount of DHW usage.  The 4-ton Nordic geo-exchange unit will mostly be running in the winter to heat our in-floor radiant system.  No A/C for now.  So when will the geo-exchange desuperheater be making the most DHW? Or to put it another way: When, if ever, would a solar thermal system be producing DHW that would go unused, since the desuperheater is handling the load?

Will the geo-exchange not run much in the summer, meaning that the desuperheater won't produce much heat?  Or will the desuperheater unit not produce much DHW in the middle of winter when the geo-exchange is working full out to  just to heat the house.  I'm a bit unsure of when the desuperheater is of most benefit.

Thanks in advance for any responses.

Well, after thinking about it for a while, I have the following analysis.

Solar thermal hot water and drain heat recovery systems are supplemental heating systems. Ultimately, you need a primary system to heat the water.

There are desuperheater systems that have some extra valves and can heat enough water to supply an entire house. Since the COP of a heat pump is normally ~3, and a desuperheater basically heats a holding tank, the cost to run such a system is probably roughly the same as a gas fired holding tank hot water heater. You're going to have heat losses from the holding tank, and gas is generally about 1/3 the cost of electricity per BTU delivered. If you have geothermal air conditioning, you do get a fair amount of heat 'for free' from the desuperheater system. However, since geothermal heat units are not as mass market as conventional systems, they tend to cost more than comparable conventional technology.  All those extra piping and valves and controllers are going to be expensive to purchase and to repair when they break.

To wrap up my long winded explanation : for a primary heat system, a natural gas fired on demand hot water heater is probably better. Compare the extra cost of adding a desuperheater to your geothermal system to the costs for a natural gas heater. Takagi makes some very nice systems for between $600 and $1500, and they can accept pre heated water from a solar array or drain heat recovery.  Moreover, on demand natural gas heaters are a much more common technology, and it's going to probably be cheaper to buy replacement parts and to repair it.

That's for primary heat. For supplemental heat : a small solar thermal array and drain heat recovery system are probably both cost effective to add in your case, in a big house with 6 adults. Just have to carefully size each system, and all the drains from the showers and hot tubs should be combined to pass through 1 drain heat recovery exchanger.