Dare to Compare
November 30, 2011
Solar thermal installations are not new, which is no surprise to a group of contractors and installers who were around in the mid 1970's through the mid 1980's when the combined state and federal tax credit was 55 percent. Our firm installed 50 or 60 systems during that time frame. There are records that indicate the concept of using solar radiation in passive building design was well understood as early as the fifth century BC.
When California was promoting solar thermal installations in the mid 1970's, there was a booklet showing solar thermal installations were common on the west coast and Florida in the mid 19th century. The first commercial solar hot water system was patented in 1891!
Let's stipulate, rather than argue why, that solar thermal works and works well. There are simple systems and as time goes on increasingly more complex systems designed to overcome some facet of solar thermal design and operation which only becomes apparent after enough data comes to light (freezing for example). That is not an indictment of any solar design, material, or operational strategy. It is an observation that over time we learn what works and what we think will work better.
Once again, without going into a lot of detail as to how OG-300 systems are rated, (you can look it up online) one of the bits of information included in that rating is called the Solar Fraction, or SOF. The solar fraction is the amount of energy provided by the solar technology, divided by the total energy required. For example if you were spending $400 per year to heat water and you installed a solar thermal system that reduced that cost to $100, the SOF could be computed at 0.75. The SOF on the same system will vary based on geographic location, climate and load. I think a good comparison is when you look at an automobile you see the MPG and understand the estimate will vary based on where you live, how you drive and what grade of fuel you use.
At minimum, the solar fraction achieved by a system must be 0.5 to qualify for the current federal income tax credit (HR 1424/ ARRA 2009).
The least costly professionally installed solar thermal system I am aware of is about $5,500 and has a 0.5 SOF when installed in the most favorable locations.
Perhaps another way to look at the SOF is the amount of reduction achieved in your water heating costs through the solar installation. Which raises the question in my thinking: could a similar reduction be achieved some other way? And if so, would the cost be similar?
To begin this inquiry you have to determine what the load is, what temperature rise is required, and what the annual energy cost is in either therms or kWh.
The load, made up of showers, baths, clothes washers and dishwasher (based on DOE testing intended to represent the typical household) equals 65 gallons.
Temperature rise from 50 degrees Fahrenheit to 120 F is 70 F. (Note 65 gal/day = 23,725 gals/year).
Sixty-five gallons times 8.3453 pounds/gallon = 542 lbs x 70 F = 37,971 BTU (if the gas storage water heater was 100 percent efficient, which we know it isn't).
Let's say the heater is two years old and has an efficiency of 0.56. The actual energy use would be 37,971/.56 = 67,805 BTU/day x 365 = 24,748,825 BTU.
That equals 247.49 therms/year for hot water and, at $1.41 per therm (2010 national average), this equals $348.96.
If the heater is electric you still use the original 37,971 BTU. The efficiency on a typical electric storage tank heater is around 0.9. The actual energy use would be 37,971/.9 = 42,190 BTU/day x 365 = 15,399,350 BTU. There are 3,412 BTU per kWh. 15,399,350/3,415 = 4,513 kWh at 0.15 / kWh =$676.95.
Most residential hot water usage is from showers and clothes washers. Changing from "standard" to a tier-2 high efficiency clothes washer will save about 4,800 gallon per year of hot water. Changing 2.5 gpm showerheads to 1.5 gpm heads and a seven- minute shower (x3 showers/day) will save another 7,665 gallons of hot water.
I used the 2.5 gpm flow rate because that is the Federal maximum. However, based on personal experience with customers, what students have told me and just plain common sense, if you think someone who wants more than 2.5 gpm can't fool, fiddle and diddle a shower head to get flow rate of 4 or 5 gpm you're not paying attention. And at the risk of being banned from sharing information in this forum again, I bet you did it to your own showerhead. If so and you install an unmolested 1.5 gpm showerhead you energy (as well as water and sewer) savings will be even more than projected here.
The total hot water saving from just those 2 fixtures equals 12,465 gallons, which is more than half of the original hot water use. I'll run through the math lower on the page but it sure seems using high efficiency fixtures/appliances has the potential to save just as much energy as an economy solar thermal system with less cost, less maintenance and quicker pay back.
This isn't my first rodeo so I understand some of the difficulties with what I am suggesting. The first issue is for these numbers to work out, the users' behaviors can't change. In other words, they can't start taking 20-minute showers. Another, and perhaps more challenging problem, is can you find a showerhead your client or perhaps you will be satisfied with? I can tell you from personal experience I have found several 1.3, 1.5 and 1.6 gpm heads that were more than adequate for the task. That doesn't necessarily mean you or your client will be satisfied but if they want to save energy I believe they can find a head in that range that will do the job.
I am also aware of some push back on the horizontal axis many high efficiency clothes washer are constructed with. Did you know there are high efficiency clothes washer designed as top loaders?
I am not suggesting you try to convince a client who wants to install a solar thermal system to do high efficiency showerheads and a clothes washer instead. But I am suggesting you at least be open to and explore other opportunities as you work at growing your business opportunities.
Math to support HE vs. solar thermal: water use reduction 12,645 x 8.3453/lb = 104,024 lbs x 70 f = 7,281,680 BTU /.56 = 13,0003,000 BTU = 130.03 therms, which is a 52.5 percent reduction in hot water usage. (You also have water and sewer fees saving of about $100).