Using Sand to Store Solar Energy
Assessing the controversial claim that solar thermal heat gathered in summer can be stored in sand for winter use
John Klingel's question was simple enough: what's the best way of heating up a thick bed of sand beneath a concrete slab with PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. tubing? But the underlying issue — whether a sand bed is a good idea in the first place — quickly takes center stage in this Q&A post at GreenBuildingAdvisor.
Klingel plans to include a 2-ft. thick bed of sand between his concrete slab and a layer of rigid foam insulation. The sand is a heat sinkWhere heat is dumped by an air conditioner or by a heat pump used in cooling mode; usually the outdoor air or ground. See air-source heat pump and ground-source heat pump., but Klingel isn't sure where the PEX tubing should be located for the best result. Nor is he sure what diameter the tubing should be, or what the spacing of tubing in the sand will work best.
Some writers think a sand bed is a waste of time. Others report they've had good luck with them, even in extreme climates. That discussion, similar to an exchange on the Q&A forum last year, is the subject of this week's Q&A Spotlight.
Forget the idea — it won't work
Count GBAGreenBuildingAdvisor.com senior editor Martin Holladay among those who think that an insulated sand bed doesn't add much to solar design. "Here's my opinion — subject to revision when someone gives me good monitoring data to contradict my statement: you can put the PEX wherever you want, because these systems don't really work," Holladay tells Klingel.
To get a useful amount of heat from the sand during the coldest months of the year, he says, it must be hot enough to get water in a hydronic heat distribution system to at least 100°F. And that, he adds, just isn't going to happen.
"The sand doesn't get that hot — or if it does, it doesn't stay that hot from early September (when it is likely to be hottest) until mid-November (when you begin to need it)," Holladay writes. "Moreover, the pumping energy is a big energy penalty — parasitic energy that needs to be considered when analyzing possible benefits. Finally, the capital costs of all those extra solar collectors is high — an investment without a significant payback."
Indeed, keeping the sand warm enough when solar energy is not enough is also on Klingel's mind. "My plan, if I go this way, is to heat the sand with the wood gasification boiler as well as passive solar," he says. "However, when the wood boiler is not running and solar is not enough, my propane boiler, which will heat the slab, is going to have to work its butt off."
He adds, however, that the temperature of the sand probably won't have to be that high because it won't be used for an active heating system, just storing heat for the night, or at most for a couple of days. Presumably the sand would gently warm the slab without an active distribution system, and in this case he thinks a temperature of between 80°F and 85°F would be enough.
You're missing the point, an advocate says
To Thorsten Chlupp, whose description of his SunRise Home is the subject of a separate Q&A post, Holladay's reply is an "apples vs. oranges" conversation.
"If you want to be able to capture and utilize passive solar gain there is in my experience no better way of doing this then with adding INSULATED mass to the foundation," he says. "A layer of sand between the slab insulation and the actual concrete slab is the cheapest and most economical way to do so. I added 180 tons of mass for $380 in material costs at the SunRise home."
The sand bed does three things, according to Chlupp: provides an insulated heat sink for passive solar gain; keeps all of the under-slab plumbing in a conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. while minimizing thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. ; and, as an option, can provide active heat storage if secondary solar heat lines are installed.
"Anyways, I argued these points before and probably will do so many more times as it seems an alien concept for many," he says. "I have independent data collection on my system and that might bring a bit more weight to this argument in the near future. To me at least the fact that I have not actively heated my home since 02/16 besides passive solar gain in temperatures well below freezing kind of proves that this concept works fairly well if it is implemented right.
"I never even had to load my sand bed actively this spring as it stores the sun's energy so well that my slab is over 70°F - my kids run around barefoot all day on it, nice and comfy... Passive solar energy is the only free energy there is and should be the very first source we should always tap into in a heating climate."
He promises Holladay to send data as it becomes available, but points out also that the sand beds are "strictly passive heat sinks." Once heat is dumped into the sand, it moves toward cooler a surface all on its own. "There is no control, nothing to extract," he says.
"Heat in - heat out by temperature differentials is all there is to it and it takes some figuring out on finding your comfort zone as it functions and reacts very slowly. If you're trying to use it as an active storage - like a rock heat bank - I agree with you, it makes no sense and is not feasible. "
Even so, the plan has holes
Be that as it may, says Holladay, but there are still some problems with this approach. First, where do you put the sub-slab insulation, and, second, how do you control the flow of heat from the sand to the slab?
"If you have no insulation between the slab and the sand, and you are dumping solar heat into your slab in July and August, then heat will flow from your hot sand to your slab in July and August," he says. "That may work in Fairbanks, but it won't work in any climate where summer overheating is a potential problem."
Also, if you're trying to keep the interior of your house plus tons of sand warm, you'll need that much more heat input. That's not an issue if heat is free, but you should factor in the cost of solar collectors plus pumps, controls and electricity to operate the system: "Bringing tons of sand up to temperature, and maintaining the sand at an elevated temperature, takes heat. If I build a house without that heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load., my annual heating load will be less than yours."
More research would be helpful
Chlupp has been experimenting with insulated thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. for years, and concedes that it's not easy to incorporate the feature into energy modeling. Nor is there much research available. Yet he finds the system works.
"I walk away from my house at 40° below and come back in the spring and don't worry about it," he says. "Sand and slabs can be a very bad idea but if designed right they can also offer great benefit from my experience...and it is my firm believe that we need to look at insulated internal mass in cold climates carefully as it can help us to make our buildings function better. I will leave it at that."
An expert opinion
This week GBA invited energy expert Mark Sevier to comment. A former employee at the Building Science Corporation, Sevier designed, built, and lives in a net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. home outside of Boston, Mass.
Mark Sevier writes:
“Martin is right. Martin's perspective is most accurate as it relates to 'direct thermal storage' systems (ones without heat pumps) - long-term thermal energy storage systems should be subject to monitored skepticism, as the numbers on paper don't work on a straight heat-loss basis. There are a lot of hours between summer collection and winter loss, and in general thermal storage of building materials inside insulation can be measured in days, not months. Huge masses of water or sand might make weeks, but a wise person starts to consider that things have gotten out of scale and a house has turned into an expensive experiment more than a cost-effective place to live.
“An example to prove the point. Take for example a 5,000-gallon reservoir of water, a 6,000 HDDThe difference between the 24-hour average (daily) temperature and the base temperature for one year for each day that the average is below the base temperature. For heating degree days, the base is usually 65 degrees Fahrenheit. For example, if the average temperature for December 1, 2001 was 30 degrees Fahrenheit, then the number of heating degrees for that day was 35. climate, and a house heat-loss coefficient of (UA) 100 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h (which is unrealistically small for most buildings). Heating the building for the season will require 6000 HDD x 100BTU/h x 24h, or 14.4 Million BTU/yr. Water stores 1 BTU/lb-F, so 5,000 gallons store 41,650 BTU/F. Now divide 14.4 million BTU by 41,650 BTU/F, and assuming zero undesired tank losses, the water will need to be 346 F hotter at the beginning of the season than the end of it, unless there is a phase change (which there would need to be). This doesn't work, and a 200-300 BTU/h load is more reasonable for most efficient houses, doubling or tripling the seasonal storage requirement.
“More realistically, a 5,000-gallon reservoir connected to solar collectors might be running in the 180 to 80 F range (note that you'll have 180 F when you don't need heat, and 80 F when you do), or about 4.16 million BTUs 'peak to valley' storable in said tank, about 1/3 of what the really efficient building above would need. Of course the sun will shine during the winter, and your solar thermal system will run (unless it's dark), but it will be running on daily energy by mid-winter, not seasonal energy, since the tank will be depleted by 1/3rd or 1/2 of the season (not considering tank losses closely, which will result in overheating the house in early winter, and further shortening of the 'seasonal' nature of the tank since overheating leads to larger hourly loss).
“The only monitored-to-work seasonal storage system that I've ever heard of was MIT Solar 1, built in the 1930's - it had a tripled glazed collector for a roof on a 500 sq. ft. building with an 18,000 gallon tank in Boston's 5500 HDD climate. Otherwise, I've heard conceptual anecdotes with data gone missing.
“For seasonal storage you need a heat pump. My observation is that for 'seasonal storage' to realistically work (i.e. not an expensive experiment), you need a heat pump. I followed someone's conceptual sales job, and I have a large solar thermal system and seasonal storage experiment I've already committed to, but I can see that using outside air is the best / cheapest approach toward seasonal storage - an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. (or ground source works too, if you like putting pipe in the ground and have extra cash to get rid of).
“What really works is a grid-tied photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. system. Using solar thermal systems for space heating is not a good investment, since that expensive hardware sits idle through the summertime, and then likely can't meet the load in the winter. Solar thermal panels can collect more solar energy per square foot than PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. panels (as much as 2 to 4 times), but they need a load to work against, which they don't always have - if the tank is already hot, they collect nothing. Grid-tied PV panels, on the other hand, can always send energy back through the electric meter, storing that energy with no losses in their power bill until it is needed in the off-season. PV panels + a heat pump operating with a COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. of 2-4 end up equaling the solar collection efficiency of solar thermal panels, and no losses from lack of summer load or huge storage tanks to buy and install.
“So, my suggestion to anyone with limited financial resources wanting to have a solar-powered back-up heating system is to install a properly sized grid-tied PV system and air-source or ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures.. For the off-grid folks, go with firewood and solar panels for DHW and electricity needs. No expensive hardware sitting idle = well invested money. Solar panels sitting idle = lousy investment.
“It’s not that passive solar doesn’t work. I like passive solar things more than the next person, but I realize passive solar isn't for everyone nor every site - privacy, glare, overheating, management of window insulation, condensation on windows, etc. are all issues that come with it. More often than not, the passive solar buildings I ride by have been largely defeated due to some issue unrealistically considered by an enthusiast. The enthusiast will live with their choices, and 'sail their ship,' but there aren't so many people in this group. Most people seem to be in the 'set it and forget it' camp, since they have lives beyond meeting home heating needs.
“My experience has been that the most important factor in the anecdotal success of uncommon systems is how much the advocate has invested in them, both financially and reputation-wise. I built a system that I subsequently figured out didn't make the most sense - why can't others discuss the shortcomings of their experiments? People must like story-telling more than science.”
Mark Sevier, PE
- Fine Homebuilding
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