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;
- 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.
- 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.
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.
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.
WASHINGTON, DC: The installation of a solar water heating system was completed today at an apartment building on Tunlaw Road in downtown Washington, DC.
The solar thermal system was designed and installed by Solar Energy Services, Inc., a Maryland- based solar installer, in partnership with Skyline Innovations, a DC-based third party solar developer.
The system includes 58 flat plate solar thermal collectors, a 3,200 gallon solar storage tank, and a 200 gallon buffer tank. It is expected to greatly offset the building’s natural gas water heating system.
“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.
ANNAPOLIS, MD: Solar Energy Services, Inc. completed the installation of a series of solar thermal systems across 11 housing complexes owned by the Annapolis Housing Authority.
SES was engaged by third party solar developer, Skyline Innovations, to design, install and commission the solar thermal systems that will greatly offset the conventional gas heating systems.
Collectively, the solar water heating systems consist of 180 panels split between Harbor House and Glenwood Highrise, two of HACA’s six public housing communities, which together house approximately 2,000 people. The solar system is expected to offset utility costs by around 30% annually.
WASHINGTON, DC: Solar Energy Services, Inc. installed and commissioned a solar thermal system at Tiber Island, a waterfront condomimium community in Washington, DC.
SES was contracted by Skyline Innovations, a third party solar developer, to design and install the solar water heating system that acts as a pre-heat to the facility’s natural gas water heating system.
The solar system consists of 76 solar arrays mounted to the building’s roof. Each array has 30 evacuated tube collectors that are closed-loop plumbed to a 5,000 gallon solar storage tank.
The system is expected to offset a large portion of the facility’s conventional utility bill.
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. completed installation of a fully automatic solar water heating system at Park Tower, an apartment building located in the historic Adams Morgan neighborhood.
The building is home to 125 apartment units throughout its five floors. SES was contracted by Skyline Innovations, a third party solar developer, to install and commission the solar thermal system. It consists of 48 solar thermal panels and three 1000 gallon thermal storage tanks. The system is slated to offset a huge portion of the building’s conventional water heating system.