Maryland Green Power Co

Solar Water Heaters: The Smartest Renewable Energy Choice

If you had to pick between spending money on Solar PV Panels or on a Solar Water Heating System, the Solar Water Heater would give you the quickest return on your investment simply because it is more efficient at capturing the sun’s energy, thus producing the most energy per dollar invested.

Furthermore, solar water heating systems are reasonably inexpensive, as discussed in a posting on affordable solar power, we noted that with available tax credits, you can get your materials cost almost to zero (with the local property tax credit fo renewable energy added, one of our residential customers in Maryland got their solar water heating system effectively for free, but always check with your accountant before jumping to conclusions).

Note that this includes  a Federal Solar Incentive of 30%, a Maryland State solar grant of 30%, and a County property tax credit (varies by County), though  Maryland’s budget for solar incentives has already been depleted for 2009, as many other states’ budgets are getting used up too, but Maryland will contribute a 30% renewable energy grant, capped at $ 2500 in 2010 (you should get pre-qualified for the system and get in line now – the Maryland State solar grant is given on a first-come, first-served basis.)

So, your remaining question should be: Which type of solar water heater to purchase?

To keep things simple, let’s break down all prevelent designs of solar water heaters into two types of systems:

A) Vacuum-tube solar collectors:

Residential and small commercial-sized vacuum-tube solar collectors are one of the newer technologies introduced into the United States’ market. It does not suit our purpose to further break down this technology into subsets like open or closed-loop systems, integral solar water storage tank or detached, etc, because these are components shared with Flat Panel Solar Water Heaters, which we will discuss next.

The most distinguishing feature of the vacuum tube system is the solar collector, which is comprised of glass tubes in which copper piping rests in a vacuum. Proponents of the vacuum-tube collectors contend that the vacuum inside the tubes provides excellent thermal insulation, so any heat captured will not conduct back out. This makes the system effective at capturing heat in cold climates. It is also pointed out that vacuum tube collectors capture solar radiation from a wide range of angles, making positioning and tracking less important. While I agree that coatings on the glass tubes let solar radiation in and minimize the amount reflected back out, and since the vacuum tubes are round the sun’s position on the horizontal axis will make little difference in the amount of solar radiation penetrating the tube (solar flux), vertical positioning is still very important, and vacuum solar collectors, like their flat plate collector counterparts, need to be mounted at a vertical angle equal, plus or minus 10 degrees, to the installation’s latitude, in order to maximize solar flux.

A drawback of the vacuum tube technology is that the tubes get very hot, and if nothing is consuming the heat produced, a heat sink needs to be provided to cool the tubes. Whereas with a flat plate solar collector, in the worst case scenario, there are pressure relief valves to protect the system, or the plates can be drained to prevent the glycol heat exchange fluid from acidifying from exposure to extreme heat, but with vacuum tube collectors there is no option to drain the system, because that would make the overheating problem even worse.

B) The other major type of solar water heater uses flat plate solar panels as collectors, and can be divided into the following sub-types:

1) Open-loop systems: The potable, or drinkable water supply is run directly through the solar heater on open-loop systems. The drawback is that the panels cannot be drained without shutting off the potable water supply. Thus, in cold climates these systems run the risk of freezing and bursting pipes.

One very clever open-loop design in TCT’s Progressivtube Solar Water Heater. (Not shown – the image at right is an AET solar panel.) The Progressivtube panel contains a pipe which winds through the panel progressively, creating a virtual hot water storage tank where 40-60 gallons of water is stores and progressively heated, all internal to the solar panel. This technique prevents cold water entering the panel from cooling down stored hot water, as happens with typical hot water storage tanks.

2) Closed-loop systems: A closed loop containing water, or a heat transfer medium such as glycol, or a mixture of the two carries heat from the solar collectors to a heat exchanger which transfers the heat to the potable water supply or other load, such as hydronic floor heating system.

One such system already mentioned is the drainback system. This system employs a differential temperature sensor and controller which checks that the fluid exiting from the flat plate solar panels is warmer than the fluid at the bottom of the water storage tank. If the water leaving the solar collectors is cooler than that in the tank, then cold weather or another condition is pulling net heat from the panels and the system is wasting energy. Furthermore, a pipe freeze is possible. The drainback system’s pump will shut off, causing all the fluid to drain out of the flat plate solar panels into a special drainback reservoir, preventing a freeze from causing damage to the system and possibly the building.

Another interesting closed-loop system is the Thermosiphon solar collector with integral heat exchanger and hot water storage tank, i.e., the solar water storage tank is mounted at the top of the solar panel.

This eliminates the need for a circulating pump since a thermosiphon effect is created, where heat exchange fluid heated by the sun rises up through the solar panel and into the storage tank, passing through the heat exchanger and transfering its heat to the potable water, then, having cooled somewhat, sinks back down to the base of the solar collector to start another cycle.  The Thermosiphon system we market uses food-grade glycol as the heat transfer fluid and is thus able to resist freezing, and is non-toxic.

In a previous blog post, we cited sources which compared the effeciency of vacuum tube solar collectors versus flat plate solar panels, and admittedly concluded that vacuum tube solar collectors will produce more heat per unit area of real estate used, but dollar for dollar, the flat plate solar systems are the best buy.

Therefore, our next blog post will detail the components of our most popular solar water heater, built in the USA by AET.

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