Solar storage: Banking BTUs

 

BY BOB “hot rod” ROHR,

contributing writer

 

One thing becomes quite obvious when you start designing and installing solar thermal systems: you rarely get exactly the amount of solar energy you need, exactly when you need it.

 

The easiest and most practical use for solar thermal is dhw, or sdhw (solar domestic hot water). These systems are more often than not a pre-heat system designed to offset a percentage of the total dhw load. The installer or designer determines a solar fraction to what to design and assemble. The solar fraction (sf) is the percentage of dhw load that the solar can be expected to deliver. Basically, it is the amount of energy required to cover the dhw load divided by the energy the solar can contribute to the load. Determining the dhw load is easier said than done. The industry has some rules of thumb, such as 20 gallons per person per day. I think that may be a tad high. Others prefer 20 gallons per day for the first two and 15 gallons for additional family members.

 

One common sense method is to look at the hot water supply they have and ask if that is sufficient. If they have a 50-gallon 37,000 BTU/hr input tank, that’s the number.

 

Another method would be to put a water meter on the cold supply to their water heater and get a real-time figure. No need to grossly oversize the supply, but then again I have never had a customer complain of too much hot water supply. But I have under-estimated more than one job and have had to correct it on my nickel. Determine to the best of your knowledge, and maybe document what you intend to provide, or supplement with solar.

 

Back to the supply vs. demand challenge. Generally, these days, the dhw load in a typical residence is required in the morning and evening hours. It may change a bit on weekends when schedules change and clothes washing goes full blast. So really, a residential sdhw system calculates out nicely. Your solar window is open the widest between 10:00- 2:00. In a perfect scenario, you want to present the coldest possible tank to the solar array as the sun rises. This drives the efficiency to the best percentage.

 

When all the calcs are said and done, a solar fraction of 45 - 60% is well within reach pretty much anywhere in the United States. We calculate that over a 12-month period, of course. It is quite possible to get 80 - 100% in the summer months and drop back to 30% or less in the colder, less sunny months. Using actual data and software simulations, I feel confident in those projections.

 

So in a nutshell, a 60-120-gallon storage, with an appropriately sized collector array will provide the solar fraction. It also presents a reasonable payback for a sdhw system.

 

But all this gets a bit foggier when you start looking at covering heating loads with solar thermal. Right out the gate you realize you get the best, and most, solar when you need it least. You can warm a lot of water all summer long. But what are you going to heat with it?

 

So the dilemma becomes how, and how much, do you try to store. This is the challenge solar installers have always faced. It hasn’t changed in the 30-plus years I have been involved with active solar thermal systems. In a perfect thermodynamic world you could exactly match the solar input, or harvest, to the ever-hanging heating load. I have read about projects in Europe that have dhw and heating fractions right up to 100%. Examine the numbers carefully to see what type of storage capacity is required to accomplish. Snoop around www.jenni.ch to see some of the 90 - 100% sf systems they have built and installed. On one apartment installation, an 8-unit complex required 205,000 liters of storage to get that sf. If my math is correct, that is around 54,000 gallons of solar storage. Could you spec that sized insulated tank to your customers?

 

But there is some encouraging news for those interested in chasing down the solar storage challenges. Water, being the medium we all work in, becomes the best shot at storage. It’s cheap, easily stored and shuffled and a fairly good conductor of heat.

 

Here are a couple examples of how to put it to use: Cedar Mountain Solar owner Bristol Stickney writes a monthly column for Phc News. In his writings, Bristol has described a developed means to store excess solar energy in the mass of the building. Typical jobs he installs have a radiant heat component. They use a series of controls and zoning to store the energy in various zones of the radiant slabs. Care must be taken to prevent overheating the space and allowing uncomfortable temperatures in those zones, of course.

 

Bob Ramlow of Wisconsin loads up a large sand bed under the slab as a parking space for solar gained during the summer months. This can take him deep into the heating season, depending on the building loads and storage capacity available.

 

Still the most common thermal storage remains insulated tanks containing plain old tap water. Tanks are readily available in all sorts of sizes, shapes and configurations. Tanks with coils inside for heat exchange have been popular over the years. We now see tanks available with multiple coils inside for various temperature outputs and loads. With the solar thermal industry ramping up again, we are seeing high-tech approaches to tank design. Stratification chambers, lances or fiber materials are being used to encourage the tank temperatures to stratify. Several manufacturers offer tanks with a wax-like product on the top for some latent heat storage. Tank-in-tank designs are another approach. Several manufacturers install small capacity stainless steel flash tanks inside the solar storage tank. The concept is to store small quantities at elevated temperature to address Legionella concerns. Some wild stratification tanks are being built these days. Yet other brands use small ECM circs to “stack” the tank in layers. Clever concepts.

 

Tanks with external heat exchangers are another ideal way to look at thermal storage; this allows you to purchase less expensive storage-only tanks. Now you have more options for capacity and this also allows you to size the external heat exchanger exactly to the load and conditions. If or when the tank springs a leak, a new insulated tank can be installed at a much lower cost.

 

This is especially true when you start looking at tanks in excess of 120-gallon capacity. Still, other contractors limit tank size to 120 gallons to take advantage of common off-the-shelf products. They then manifold together multiple tanks to configure the exact capacity to match the design. Clever piping and 3-way motorized valves can allow you to “load” these tanks at different temperatures or to match the daily gain, providing sufficient temperatures to meet that load. Large quantities of lukewarm water doesn’t help much for covering you heating loads. Certainly, designing your heating distribution temperatures as low as possible will help maximize the solar contribution.

 

So at the end of the day, and the end of the article, there really isn’t a “one size,” or one method that fits all approaches to solar storage.

 

Research and educate yourself on the options for tanks, heat exchangers and controls. Talk to the old solar dogs with years of experience.

 

Jump at any chance to attend the Frankfurt ISH show. InterSolar in Munich is another great show to see wild and unique solar storage products. These shows and others have U.S. versions now. Most of the big players in the industry — both foreign and domestic — bring product to the plumbing and solar trade shows across the States.

 

Domestic tanks suited for solar have been around for many years and new players are entering the market with high-tech solutions. Rheem has built the external coil Solaraide for many years. Vaughn stone-lined tanks are another old name in the industry. Heat Transfer Products has some clever hybrid solar tanks, some with high efficiency back-up burners built in. Lochinvar has a clever approach called the LockTemp, available in sizes from 78 - 2,500 gallons. Bradford White offers 10 or more solar specific tanks. Heat-Flo builds dual coil, stainless steel and solar storage solar tanks. Many solar storage tanks will have a provision for a back-up electrical element. This provides a nice, simple dual-fuel option.

 

Bob “hot rod” Rohr has been a plumbing, radiant heat and solar contractor and installer for 30 years. Rohr has been a long-time RPA member, and has since joined Caleffi North America as manager of training and education.