Benefits of Solar Hot Water:

Floridians see their electric bills go through the roof during July, August and September and don’t need to be told that air conditioning uses a lot of electricity. On the other hand, while you may use hardly any air conditioning during March, April and November, that big 3,500 to 4,500-watt heating element in your water heater is burning away every day of the year. (Some high-recovery models have two heating elements.) At today’s electric rates (including fuel surcharges and utility taxes) of around 14 to 15 cents per kilowatt hour, water heating can easily cost $800 to $1,200 each year for a three to five person household. In South Florida, a solar water heater can almost completely eliminate this portion of your electric bill.

Naturally, switching to solar water heating means going green in a big way. But you might also be surprised to learn that solar water heating gives you benefits that improve upon conventional electric or gas water heaters:

* More hot water. Most electric water heaters hold enough hot water for about 20 minutes of continuous use. Then the electric element reheats another tankful. You can run out of hot water if two people are showering at the same time, or if you take a shower after the clothes washer or dishwasher have been running. Solar storage tanks are sized to store an entire 24 hour’s worth of hot water because there is no sunshine at night. But this gives you the added benefit that you are less likely to run out of hot water when there are multiple uses at the same time.

* Hotter water. The water delivered by a solar collector panel can be hotter than the thermostat setting on your electric water heater. This means you will have hotter water for the dishwasher and for showers if you want it. (A mixing valve on the storage tank protects you against scalding.)

* Plenty of hot water during emergencies. With a passive solar water heater, or a system with a solar-powered circulating pump, you will have your customary supply of hot water even during electric power outages. This is an important feature even if you have a standby power generator, because standby generators are not typically sized to handle the power consumption (4,500 to 9,000 watts) of a conventional electric water heater.

System performance

Because incoming solar radiation—called insolation—is often interrupted by cloudy or rainy weather and fluctuates in intensity with seasonal changes in the sun’s position in the sky, solar water heating systems are usually sized to collect and store an entire 24 hours or more worth of energy to meet hot water demand. A modern solar water heating system storage tank will usually have a backup electric heating element to meet unusual peaks in hot water use or extended periods of very cloudy or rainy weather.

How to find your current electric rate

How many cents per kilowatt-hour are you currently paying? Here’s how to find out:

Locate your most recent electric bill. Your utility company may break the kilowatt-hours (kWh) used into two categories: the first 1,000 kWh and the kWh used above 1,000. If this is the case and your power usage was greater than 1,000 kWh, add the second kWh figure to 1,000 to find your total kWh for the month.

Next, divide the total kWh used into the total amount due for the current billing period; i.e., total including base electric rate, fuel surcharge, utility franchise fees, and taxes. Do not include any past due balances or late fees.

For example, if you used a total of 1,685 kWh and the total amount due for your most recent billing period is $244.33, your current cost for electricity is $244.33 / 1,685 = 14.5 cents per kilowatt-hour.

Choosing a Solar Hot Water System:

Direct System

Also called an “active” or “open loop” system, this is the type of system most
 often installed in the central and southern areas of Florida, and other non-freezing sunbelt climates within the United States.

                                                                                              

                                                                                                          ICS System



Integral collector storage systems, also called “batch” solar heaters, combine the
 hot water storage tank and the solar collector surface into a single component,
 eliminating the need for circulating pumps or automatic control systems. In its most simple implementation, a water storage tank painted black and sitting out in the sunlight is a rudimentary ICS system.

This type of system works best as a pre-heater for a conventional or tankless water heater. The cold water line that feeds the conventional water heater is diverted [1] and sent first through the ICS solar module [2]. Circulation is provided by utility mains pressure. In other words, when hot water is drawn out for use from the conventional water heater [3], the storage tank is replenished with solar heated water instead of cold water. This allows the electric or gas heater to work substantially less.

Advantages. The biggest advantage is simplicity: the system has no pumps, no temperature sensors, no electronic controls and no heat exchanger. When combined with a tankless water heater, the system can free up five to six square feet of floor space by eliminating the conventional water heater storage tank.

Disadvantages. The biggest disadvantage is nighttime heat loss. Stored heat is lost through the glass cover plate at night, which of necessity cannot be insulated to prevent heat loss. However, this heat loss is reduced in advanced ICS systems by stretching a thin clear film just underneath the glass cover plate, which creates an insulating air gap. Also, while the greater thermal mass of stored hot water within an ICS solar module makes this type of system more freeze resistant than the direct system (above), ICS systems are not appropriate for climates that experience more than four to five freezing nights per year.

Drainback System

Drainback systems have a few more components, but are specifically designed to
 provide fail-safe operation in climates with frequent freezing during the coldest
winter weather. The solar collector and control system are the same as the direct circulation system. However, an antifreeze solution is circulated through the solar collector and back into a heat exchanger in the hot water storage tank. The addition of a heat exchanger adds to the cost of the system and creates some degree of heat transfer energy loss, and this combined with the fact that these systems are typically installed at higher latitudes, where incoming solar radiation is reduced, makes indirect solar water heating less viable than its direct circulation cousin.

                                                                                

                                                                Questions about Solar Hot Water

Which home energy uses are the most expensive?

The pie chart shows where the money goes in a typical Florida family’s annual electric bill. The chart is based upon an electric power consumption study by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. The “typical” Florida household and energy use pattern shown above is based upon these factors:

·         Florida home built in 1990 with 2,400 square feet of living area

·         9.5 SEER air conditioner with thermostat set at 81°F during day and 78°F at night

·         cold weather thermostat setting of 64°F during day and 68°F at night

·         two adults and two children in household

·         80 gallons of hot water per day

·         14 cents per kilowatt-hour electric cost (including fuel surcharge and utility taxes)

Is your hot water use average?

Actually, an active American family of four is probably closer to 100 gallons per average day and just over $1,000 per year at 14 cents per kilowatt-hour. Here is an example that shows how a four-person household can easily use 100 gallons of hot water per day:

·         three people take an eight-minute shower every day; the fourth showers 12 minutes for a longer shampoo and rinse: (4 showers x 2 gallons per minute x 8 minutes x 80% hot water) + (1 shower x 2 gallons per minute x 4 extra minutes x 80% hot water) = 57 gallons per day

·         five loads of laundry per week; for example, one load each of whites, darks, towels, sheets, and blue jeans: 5 clotheswasher loads x 32 gallons per load / 7 days = 23 gallons per average day

·         At least four face and hand washings x 1 gallon per minute x 4 minutes each x 80% hot water =13 gallons per day

·         few sit-down meals because everyone is constantly on the run, so the automatic dishwasher is only run twice per week: 2 automatic dishwasher loads x 14 gallons per load / 7 days = 4 gallons per average day

·         One shave x 1 gallon per minute x 2 minutes = 2 gallons per day

·         Hand-rinse dishes once a day x 1 gallon per minute = 1 gallon per day

If you happen to have small children, each bath averages about three gallons less hot water than a shower. However, if you happen to have two active teenagers, each taking two showers a day and perhaps creating two or three extra loads of laundry per week, the total can easily jump to over 120 gallons per day. Heating 120 gallons per day from 72°F to 140°F, at 14 cents per kilowatt-hour, costs about $100 per month.

really.

Why not use solar electric power for water heating?

This is an excellent question. Understanding the answer provides some insight into why, 50 to 100 years from now, solar water heaters will still be the best way to heat water.

And why you should go ahead and install a solar water heater today.

The issue is conversion efficiency. A typical flat plate solar water heating collector transfers about 63 percent of the solar energy that strikes it directly into the water (or heat transfer fluid, in indirect circulation systems).

A typical photovoltaic (PV) solar electric cell converts only 15 percent or so of the energy striking it into electrical energy under ideal conditions: PV cells lose efficiency as operating temperatures rise. Additional inefficiency occurs when an inverter changes direct solar electric current (DC) into alternating current (AC). The end result is that a PV cell with a 15 percent rated peak efficiency only delivers about 10 percent of the energy striking its surface to a demand load.

So here is the problem: The solar electric PV panels would require more than six times the roof area of a flat plate solar thermal collector to meet the same (water heating) demand load.

Meeting the hot water needs of an average Florida family of four requires about 40 square feet of flat plate solar water heating collector surface area, so to do the same job with PV panels would take about 40 x 6 = 240 square feet of PV panel surface area.

What about cloudy weather?

Good question. Actually, a solar water heating system will typically collect about half the solar energy of a clear, sunny day on an overcast day. If you have ever had the experience of going to the beach on an overcast day and still getting a sunburn, you will understand this phenomenon. Clouds block many of the visible wavelengths of sunlight, but much of the solar energy still gets through.

Will my water be hot enough?

Yes. Solar heated water is often hotter than the thermostat setting on your water heater. In fact, for safety reasons our systems include mixing valves to make sure the hot water going into your house isn’t too hot. On the other hand, sometimes we may have extended periods of very cloudy and rainy weather. During these periods, a backup electric heating element in your water heater / storage tank will automatically heat water to the water heater’s thermostat setting.

Will I have hot water during cold weather?

Yes. Solar water heating collectors typically deliver excellent performance in Florida during cold weather because the sky is very clear during winter high pressure waves. The glass cover plate and insulation inside the collector prevent collected heat from escaping to the outside air.

Can you use my existing water heater as the solar storage tank?

Usually, no. Solar water heating systems are designed to heat and store 24 hours worth of hot water during the daylight hours, so the tank has to be large enough to store 24 hours’ worth of hot water. Most conventional electric water heaters in Florida homes have a capacity of about 52 gallons. Standard solar storage tank sizes are typically 80, 100 and 120 gallons, with 80 gallons being appropriate for most three- to four-person households. Also, solar storage tanks typically have better insulation than conventional electric water heaters, to minimize overnight heat loss.

I’ve heard that “evacuated tube” solar collectors are much more efficient than flat plate collectors. Are evacuated tubes better for solar water heating?

While it’s true that evacuated tube collectors can attain higher temperatures than medium temperature flat plate solar collectors, as well as higher efficiencies under certain conditions, research studies have demonstrated that flat plate solar collectors produce greater savings for medium temperature (120–160°F) water heating on a year-round basis, even in colder climates.

And we do not recommend evacuated tube solar collector systems for high wind zones such as Florida. An evacuated tube—or vacuum tube—collector system resembles a bank of flourescant light bulbs on your roof. Naturally, an evacuated tube cylinder will have minimal wind resistance, which is good. But it will have very poor impact resistance if struck by a flying object, which is bad. Modern flat plate collectors have tempered glass cover plates that can sustain direct strikes by such wind-driven objects as, for example, a piece of 2x4 lumber.

Evacuated tube collectors are very popular in Europe, where their high efficiency offsets more frequent overcast weather conditions and lower average air temperatures than Florida. On the other hand, Europe does not experience hurricanes.

Can my home solar water heating collectors also heat my swimming pool?

No. Home water heating systems operate at 125–140°F water temperatures. These systems typically have a relatively small solar collector surface area (10 to 16 square feet per occupant) but the solar collectors have special heat absorbing materials, glass cover plates and insulation to maximize heat gain and reduce losses.

Pool heating systems only reach 75–95°F water temperatures. This allows the use of relatively inexpensive polypropelene solar collectors. However, because even a small residential swimming pool may hold 10,000 gallons of water, a typical residential pool heating system must deliver as much as 10 times more total daily energy than a typical home water heater. So while a less expensive type of solar collector can be used for pool heating, a solar pool heating system needs a much larger solar collector surface area.

References and Notes

1. Trinkl, Christoph et al. “Performance of Vacuum Tube and Flat Plate Collectors Concerning Domestic Hot Water Preparation and Room Heating,” 2nd European Solar Thermal Energy Conference 2005 (estec2005), Freiburg, June 21–22, 2005.
2.  Heating 100 gallons of water from an inlet water temperature of 72°F to 140°F requires 56,712 BTU: 100 gallons x 8.34 pounds per gallon x 68 degree temperature rise = 56,712 BTU. Natural gas has an energy content of 100,000 BTU per therm. A typical natural gas tankless water heater is about 82 percent efficient, so it needs 56,712 BTU / 0.82 = 69,161 BTU of energy input, and 69.161 BTU / 100,000 BTU/therm = 0.69 therms worth of natural gas to raise the temperature of 100 gallons of water by 68 degrees. At a price of $2.30 per therm, the cost is $1.59 per day.
3. One kilowatt-hour (kWh) of electricity converts into 3,412 BTU of heat energy. An electric water heater delivers almost all of its electrical energy to the water, so it needs 56,712 BTU / 3,412 BTU per kWh = 16.62 kWh to do the same job. At 14 cents per kWh, the electric water heating cost is $2.33 per day. So the natural gas cost is ($2.33 - $1.59) / $2.33 = 32 percent less expensive. Of course, the daily cost of solar energy is zero.
4. According to the U.S. Energy Information Administration, the average U.S. electric utility powerplant sends about 1.36 pounds of carbon dioxide emissions into the atmosphere for every kilowatt-hour (kWh) of electricity delivered to an end-use. If an average family of four uses 80 gallons of hot water per day, this requires 5,581 kWh per year, assuming a temperature rise of 68 degrees (from 72°F to 140°F) and a 15 percent added energy factor for maintaining the temperature of stored hot water. 5,581 kWh x 1.36 pounds of carbon dioxide per kWh = 7,590 pounds. 7,590 pounds / 2,000 pounds per ton = 3.8 tons of atmospheric carbon dioxide emissions per year.