Thursday, April 26, 2012

Solar Thermal is Hardly Dead

A recent article with the confrontational title "Solar Thermal is Dead" touting Heat Pump Water Heaters (HPWH) coupled to PV systems as replacement to solar water heating (SWH) made a big splash in the solar energy and green building web-o-sphere. This piece was a valid thought experiment about where technology may head but misses a few key points for why solar thermal systems are still a better way to heat domestic water than a HPWH/PV system.  Here are three counterpoints:

1. HPWHs aren't a cure all 

HPWHs move heat from the surrounding environment into a tank of water through the heat pump cycle using mechanical work.  Just as with space heating and cooling, this can be a very energy efficient way to achieve a temperature goal versus traditional HVAC equipment.  There are a couple of problems with HPWHs however.  
Energy efficiency doesn't have to be about reduced quality of life
First of all, the "mechanical work" component of them makes a good bit of noise just as power-vented natgas water heaters (another higher efficiency water heating appliance) have a noticeable fan noise.

Secondly, HPWHs don't work as well if the surrounding air is cold.  Placing it in a cold basement or garage will lead to less than optimal performance that the article's author used to compare this technology to SWH.

Thirdly, the efficiency factor of a HPWH is based on it operating in hybrid (heat pump) mode.  Just as with SWH, HPWHs have a back up element (usually electric resistance heating coils). When a big call for hot water such as to fill a bathtub, might outpace the hybrid mode of the water heater to recover the volume of water required.  In this case, the electric resistance heater kicks in and the water heater becomes no more efficient than a standard tank style electric water heater.

2. The maintenance on a SWH is overstated

and the author underrepresents the maintenance on a PV system.  A trained solar water heater contractor can install a complete pressurized system as a retrofit to a home in less than a day (I've personally seen this over a dozen times). Once system pressure is reached, any solar loop leaks are immediately apparent and are unlikely to spontaneously occur in the future. Circulating pumps are a mechanical item in a SWH system but have little stress applied to them since they have minimal head pressure to overcome. These circulating pumps are also low cost items to replace if they do fail. Solar fluid replacement is a non-issue with a properly sized system since stagnation temperatures would be reach less often.  I'm not sure why the author brought of tracking as this is rapidly becoming a non-issue in both PV and solar thermal as module prices fall; with the fail rate on trackers, it isn't really something to bring up in a negative comment about maintenance on SWH anyways. 
There are companies in business just to do maintenance on PV systems
PV isn't completely free of maintenance either. Inverters are the short pole in a PV system and they are significantly more expensive to replace than any piece of a solar water heater.

3. PV takes up too much real estate

I was going to address the author's underestimate of the installation cost of a PV system. He was called out about this in the comments to the article and admitted that his 1.4kW installation price was dependent on installing a 9kW system along with it.  I think that's a huge if...
SWH array fit between architectural roof facets
In any case, this ties into my final argument for why a PV/HPWH hasn't killed SWH and won't for some time.  PV takes up way too much space to be practical for many homes.  Check out the house above that I was working on last week in Michigan. There were too many roof facets to find a spot on the south face for much more than the 3-panel solar water heating system that we installed. You couldn't fit in enough PV between the dormer below and valleys on each side to offset the energy that the solar water heater would.  For you solar techies out there, good luck on convincing either architects or home buyers that plain expanses of roof are aesthetically pleasing; architectural roof elements aren't going anywhere so solar will have to work around these.
Tale of 2 arrays.  The SWH collectors on the left produce the same amount of energy as the PV on the right.


I'm a big fan of all things solar so I don't want to come off as a PV basher. My point is to highlight that there are appropriate applications for certain technologies but not any one thing is a solution to everything. Eventually PV may fall so low in price and become much more space efficient thus relegating SWH to relic status. This hasn't happened yet. Heating water with the sun will continue to be best done with a solar water heater for many years to come.
Let's keep the intra-solar squabbles to a minimum--we have bigger enemies to address