Solar HW vs. PV Electric - where would you invest your $$?

Paul B_5

Regardless of what the placement is (of the Hardware )or what its competing against .(mod con) Anything that gives you free heat energy once the hardware is in place is going to be a Win situation for the future. Stick with solar hot water first as its 60 - 90 % eff. and maybe some 18% eff Pv's in the future to offset some of your On-Peak electrical loads.Of course the Hot water stuff being the cheapest to start with first anyway and the quickeset payback.
Re : your question on Pv decay.. a good brand name panel will produce + or - about 5% of rated output for the 25 year warrenty that most have, and will obviously degrade but its a long ways out. ( 50 year old solar panels are still producing power).
Re ; panel placement..solar collection devices Need good sun as to Wind turbines need good wind to be worthwhile..
Don't get burn't by Maybe or I think I'm ok only to find out what youv'e bought or installed apppears to be a Lemon and the devices underperform and it rubs the solar industry the wrong way....Paul

Jason Horner_2

Just wanting to stimulate some discussion about the relative merits of investing in current technology PV electric systems vs. solar HW systems.

This is a question that I get fairly frequently these days, and while I know that a good portion of the answer is "It depends", there are a lot of factors which can turn us in one direction or the other - especially when money is a factor.

We have issues like radiant heating vs. DHW use;
Does solar HW make sense with a 94% modcon installed vs. PV electric;
What is the efficiency decay of PV vs. solar as the collectors age, and so on.

Can either make economic sense when you only have a small roof area which gets decent sun exposure, as is the case in many smaller city homes (say 10 x 20 when you factor in dormers, plumbing and ventilation penetrations, etc... and no opportunity for backyard placement) - where does it begin to become economic?

Love to hear what everyone is thinking.

Robert O'Brien

DHW

has a quicker payback than PV,with a lot smaller investment

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CC.Rob_7

recent HO experience

We looked long and hard at 3 options, and have recently signed a contract for the option we chose.

Background. We have a recent 88% gas-fired Burnham Revolution serving 2 zones of fin-tube and a functional, but IMHO poorly performing indirect. (Excellent insulation, but a small, tight 3/4" coil and a crummy aquastat.) Full ODR with indoor feedback. Have spent a good 4-5 years working on the building envelope and electrical loads to the point that the next logical step was active solar in some form. (On a design night of 5F, our fin-tube will briefly see 135F supply water; the rest of the year it's 85-120).

The options we looked at were:

1) SDHW + space heat. Drainback, big storage tank, immersed coils, etc. Huge initial cost and over-the-horizon long payback given the fairly high efficiency of the current system. Would require all available roof space and the fraction of the seasonal load that would be covered with this relatively high temp fin-tube emitter system was just not appealing. Most panels would be sitting offline for 5 months a year during summer, when a PV system could be really cranking out some juice. Solar thermal is a mature technology, though, and has straightforward installation/integration.

2) PV. Have roof space for about 3.6kW that would offset about 65-75% of our annual electrical use. Even with some good incentives, the initial cost was out of reach and the payback around 12-14 years. It's also apparent that PV panel technology is still maturing, and that there may be better panels and better incentives in the next couple years. However, a grid-tie system is about as simple as it gets, install-wise, and the net-metering laws here in MA are among the more homeowner-friendly.

3) SDHW. Closed loop glycol system that will do 100% of the DHW load from about May to mid-September, and will do DHW preheat the rest of the year. Initial cost after rebates and incentives was within reach. Payback is 6-7 years, even though we already have a pretty efficient means of making DHW. Technology is very mature, installation straightforward.

In two weeks, the panels go up for option 3. Panels will be placed to leave as much of the rest of the roof as possible available for future PV.

A whole lot of learning was done here on the Wall (e.g., the frequent mention that SDHW is a no-brainer first step; options for systems; discussion of tubes vs. plates, etc. etc.), and proven out by more reading, research, some quality time with software like SAM, PVWatts, RETScreen, and talking to others locally who have these systems.

Been a fun trip, and we ain't done yet.