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Solar veteran Roger Perry discusses the pros and cons of One Tank Vs Two for Solar Back-up

All solar water heaters in Maryland, D.C. and the mid-atlantic provide a family of four with around 75% of their annual hot water load. Most customers use their existing electric or gas sources for the remaining 25% backup (usually needed dead winter). When installing a solar system one question that will come up is “Should I use my existing tank and have it fed by the solar system?” or “Should I remove my existing tank and use the electric back-up that comes in the solar tank?”. A couple of situations make this an easy decision;

1. An existing electric water heater with no room for another tank. This is especially satisfying and cost effective if the existing tank is leaking or on it’s last legs. It’s like getting $1500 off the cost of a solar system because that money would have needed to be spent anyway. In this case you would definitely chose a single tank system.
2. The other is if you have a gas water heater. While not as cut and dry as the example above, using the electric element would mean using a higher cost fuel for back-up (not so much with propane). Much of this extra cost would be mitigated because the single tank back-up would not run as much because it would be affected by solar input without running a faucet. You would need to run an electric circuit (30 amp, double pole breaker). This may be difficult or very easy depending on the breaker box location and available space in it. Most jurisdictions will also require a master electrician and a permit for this to be done. In this case, I think, most people would shy away from a single tank system unless there was just no room for another tank.

Let’s look at the pros and cons of each system;

SINGLE TANK PROS

• Smaller footprint
• Less heat loss
• Electric element can sense the solar output and not come on from stand-by losses
• Can go “all solar” just by turning off the element (essentially flipping a switch).

SINGLE TANK CONS

• Less back-up in cloudy weather (can be compensated with a larger solar tank which will have a larger back-up capacity).
• Less solar storage when the element is on (can be compensated for with a larger solar tank which will have more solar storage).

TWO TANK PROS

• Usually greater back-up capacity

TWO TANK CONS

• Greater heat loss
• Larger footprint

The secondary backup tank can’t sense the primary solar tank temperature unless a faucet is turned on, sending the water through the two-tank system.  This causes the second tank to turn on from stand-by losses when the primary solar tank is already plenty hot.

Going all solar requires operating valves as well as turning off back-up.

Personally I’m a fan of single tank systems. For the most part their two main drawbacks can be compensated for by installing a larger tank. An upgrade from a 80 to a 120 gallon solar tank is only a few hundred dollars. For a 50% increase in solar storage it is a small price to pay.

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[vc_row][vc_column][vc_column_text]WASHINGTON, DC:  Skyline Innovations, a third party solar developer, engaged Solar Energy Services, Inc. to design and install a solar water heating system on an apartment building on MacArthur Blvd. in downtown Washington, DC.

The solar thermal system will include 20 thermal panels on the south facing roof of the building, as well as a 1,000 gallon solar storage tank.  The system will act as a pre-heat to the building’s existing gas water heater, reducing their utility costs by around 30% annually.[/vc_column_text][/vc_column][/vc_row]

[vc_row][vc_column][vc_column_text]FREDERICK, MD:   Solar Energy Services, Inc., (SES) a leading solar energy products and services provider, announced today that it will begin work this week on a large solar water heating system for the Frederick County Adult Detention Center in Frederick, Maryland.

The renewable energy system, equivalent to approximately 130kW of power, will consist of 57 Apricus (AP-30C) evacuated tube solar thermal collectors, collecting thermal energy to deliver to the building’s water heating system. With each collector holding 30 tubes, the completed system will generate more than 2,500 gallons of hot water per day for the facility, using a total of more than 1,700 solar tubes.

SES Vice President and founder Roger Perry noted that prisons make excellent applications for solar water heating. “The occupancy is constant and all the hot water needs for laundry, bathing, cooking and cleaning are substantial.  Aside from the environmental benefits, this system will pay for itself in a short period of time.”

The solar project is funded in part by ARRA funds, but also from the sale of Solar Renewable Energy Credits (SRECs). In 2011, Governor O’Malley signed a bill into law to include solar water heating in the state’s solar goals, thus allowing solar water heating systems to benefit from the sale of SRECs.

About Solar Water Heating

A very mature solar technology, solar water heating is 3 to 4 times as efficient as solar electric (photovoltaic) technology and therefore can be very cost effective where there is a regular, daily requirement for hot water. Residential, multifamily, prisons, hotels, dormitories, military barracks, industrial processing, health clubs, car washes and restaurants are all very good candidates for solar water heating systems. The solar systems are designed to provide preheating for all domestic hot water, allowing the traditional heating source to provide any supplemental thermal energy as required.

About Solar Energy Services, Inc. and Solar Water Heating:

With more than 30 years of experience, Solar Energy Services (solarsaves.net) is a leading solar firm in the mid-Atlantic, with a unique expertise on commercial scale solar water heating. Based in Millersville MD, SES provides design, installation and service for commercial, institutional and residential applications.[/vc_column_text][/vc_column][/vc_row]

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“If my collector is at 136 degrees, my TST (bottom of tank temperature) is at 161 degrees and my S3 (top of the tank temperature) is at 154 degrees – why is my circulator pump still running? Won’t this cool the bottom of the tank?”

SES says: The pump runs intermittently for a minute periodically when the collectors reach 240 degrees. This keeps the collectors from overheating. Your maximum tank temperature is set to 160. If the bottom sensor, the TST reading, exceeds 160 degrees, the controller will run the pump briefly in the evening to bring the tank temperature back down to 160.

It has been our experience that setting the max temp higher than 160 leads to overheating problems. Yes, you are losing a little heat from the tank when the cooling feature comes on, but at this time of year it is excess heat. You are producing much more hot water than you are using at this time. This is the summer solstice; the collector and tank temperatures will moderate in a few months. Some customers add an extra loop to their solar tank and use that excess heat to heat a swimming pool or hot tub. Most just use control features to manage the excess heat.

Don’t worry about the power consumption. The Grundfos Alpha pump only uses about 5 Watts whether heating or cooling the tank.

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WASHINGTON, DC:  Earlier this week, Solar Energy Services, Inc. completed the installation of three separate solar water heating systems on the campus of American University…

 The Mary Graydon Center, which houses a dining hall as well as various administrative offices, was outfitted with a rooftop solar thermal collection system which included 16, 30-tube solar collectors.  These collectors are closed-loop, plumbed to 7, 120-gallon pressurized tanks for a total storage capacity of 8,406 gallons.

Anderson Hall, the largest of AU’s dormitories housing over 770 students, was outfitted with 81, 30-tube solar collectors closed-loop plumbed to a unpressurized 5,000 gallon storage tank.

Letts Hall, housing dormitories and a fitness center, was outfitted with 55, 30-tube solar collectors closed-loop plumbed to 1, 3,111 gallon storage tank.

All three solar water heating systems will serve as a pre-heat to the buildings existing conventional water heating systems.

Click HERE for post-installation pics

WASHINGTON, DC:  Solar Energy Services, Inc. of Millersville, MD has been contracted by Skyline Innovations, Inc., a Washington, DC third-party solar developer, to install two solar water heating systems at the George Washington University campus in downtown DC.

The systems include a ballasted 30-panel evacuated tube system on the flat rooftop of one dormitory and a similar 60-panel system on the flat rooftop of another dormitory. The 30-panel system design will utilize an existing 2,500 gallon storage tank in the penthouse for solar storage, while the 60-panel system design utilizes an existing 2,500 gallon tank in the basement mechanical room.

Click HERE for post-installation pics

WASHINGTON, DC:  Installation of a 14 panel solar thermal system was completed at an apartment complex in historic Adams Morgan earlier this week.

Solar Energy Services, Inc., a Maryland based solar contractor, was engaged by Skyline Innovations to design and install the solar water heating system that will act as a pre-heat to the 33 units that use the building’s conventional heating system.

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Crosstown Properties, LLC – Multifamily Solar Water Heating (Completed July 2010) – Washington DC

SES was engaged by a third party solar project developer to install a 32 panel (1280 SF) solar water heating system for an occupied apartment building in Washington D.C. The system included the installation and integration of 1600 gallons of additional solar storage in non-pressurized tanks. The system is designed to offset approximately 70% of the water heating load for this 45 unit apartment building. The solar developer provided all of the capital to install the system at no cost to the building owner. The building owner receives clean energy at a discount to their traditional natural gas, achieving a monthly savings immediately upon commissioning of the system.

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HAVRE DE GRACE, MD:  Solar Energy Services, Inc. just finished installing a solar water heating system at Laurrapin Grille in Havre de Grace, MD.  The system includes two, 80-gallon tanks and four, 4′ x 8′ solar thermal panels on the southerly facing roof.  The system is expected to offset the restaurant’s hot water load by at least 50%.

COLLEGE PARK, MD:  Earlier this week Solar Energy Services, Inc. completed the installation of two separate solar energy systems at the Cherry Hill Campground.

The solar domestic hot water system consists of 10 panels, for a total of 400 square feet of collectors integrated with four, 120 gallon solar storage tanks, each with dual heat exchangers. On the same roof, SES installed a 22-panel solar pool heating system (920 square feet) designed to heat the main swimming pool during the spring and fall, saving Cherry Hill Management a tremendous amount of energy and money.

As part of the installation, SES also replaced the existing inefficient boilers with high efficiency modulating, condensing boilers (natural gas) integrated with the solar system to provide backup energy for domestic water heating. SES also installed crossover valves to allow Cherry Hill to divert solar energy to the secondary pool when the main pool reaches desired temperatures.

Post Install Info and Pics