Packing Heat

I’d venture to guess even the brightest know it all gurus on this thread have tried and failed at energy projects along their careers😏 Pushing boundaries is one way to learn and expand.

It’s your money, your property you get to try whatever you want.

Solar thermal is not fact not dead, doubt it ever will be.

I spoke with Sun Earth at AHR a few weeks ago, plenty of projects still be designed and installed. New technologies are being applied like the Viessmann ThermalProtect absorber coatings.

True the residential boom once again came and went, like the two previous.

The only solar thermal heating besides pool heaters that I've seen operating for years is direct air heating. I forget the name of the company. It blew solar heated air into industrial buildings where ~60° sufficed. When sun did not shine then good old NG kicked in. If your entire roof is covered with solar panels then you save something on your A/C use. Otherwise Contrarion & Kaos observations match mine.

4- 400W modules, 15-18% efficiency, produce maybe 6 kw over course of a day . Depend on the tank size, that will be a slow go to heat or recover a tank of water. PV modules degrade over time, thermal do not.

I do agree with the simplicity of the Sun Bandit PV to thermal, both parts and installation.

No, I get your point on the math. The cost is in the eye of the beholder. Statements like solar thermal is dead really does little to help the industry. It will be dead when the sun no longer shines I suppose😮

There are pros and cons to both methods of generating DHW from the suns energy.

The selling feature of PV to DHW is that it didn't involve connecting into the grid, no interaction with the utilities, engineered permission, licensed electrician obtaining permits, etc. It is attractive as a stand alone system, somewhat installer friendly for the average handy homeowner.

Unique to HH is most of the folks are capable of doing their own installs. Some with the tutoring of the experienced talent present here.

I'm not sure discouraging someone from trying something different based on what the should spend or ROI, is true to the motto of HeatingHelp?

Has everyone of you projects penciled out financially?

Present out the concepts, the thermodynamics, the physics, the math, let digest the various opinions, then decide how to spend their $$.

@Larry Weingarten : "As thermal captures roughly four times the energy per square foot of collector…"

I've seen that claim thrown around. I went a long way down the road with evacuated tube collectors (which at the time were the most efficient available) about 15 years ago, even going to the point of calculating detailed month by month estimates of production. The output of solar thermal is highly dependent upon outdoor temperature.

In my climate (Washington, DC) in July, when the average daytime high is 88.3F, the collectors have an efficiency of 62% with a water temperature of 120F. The PV panels that are available today typically have efficiencies in the low 20s, so that's maybe three times as efficient.

However, in January, when the daily average high is 42.5F, the efficiency of solar thermal is only 46%, so about twice nominal PV.

Conversely, the efficiency of PV is highest when the temperature is low, and drops as the temperature rises. PV panels typically have a temperature coefficient around -.4%/C, so with a 46F (25.6C) change between July and January you get about a 10% increase in efficiency. Nominal efficiency is measured at 25C /77F, so in January you could well be getting 30% or more efficiency from PV panels. Thermal is still more, but now it's only 50% more.

Where I am there is 3.3 times as much sunshine in June as in December, but a thermal solar collector produces 4.1 times as much in July as it does in December because of the effect of temperature. (June has more sunshine than July, but July gives more heat because is warmer out). PV on the other hand produces about 2.2 times as much in June as in December, due to the temperature effect.

As I wrote upthread, the biggest problem with solar thermal is trying to size the collectors to deal with the seasonal variation in output, and then dealing with either the shortfall or the excess. Even if you just use PV as a replacement for solar thermal, the temperature effects make that a smaller problem. But there's also a fundamental difference, in that the surplus that a PV panel produces is electricity, which is useful in all kinds of ways, as opposed to heat, which takes active effort to vent safely. And even if you don't have a use for the surplus electricity you can safely limit the production of a PV panel simply by not attaching loads to it.

Hi, "As I wrote upthread, the biggest problem with solar thermal is trying to size the collectors to deal with the seasonal variation in output, and then dealing with either the shortfall or the excess." … It required some backerwards thinking, but I got around these problems by using inefficient collectors. The inefficient part matters. Even at stagnation on a summer day, they don't exceed 170F, so no overheat protection is needed, just a tempering valve. Being NSF rated poly pipe, they can freeze without damage. So, I oversized the systems collector to storage, and this allows even a little sun in winter to quickly bring the tank up to a usable temperature. This isn't a system for New England. I've measured temps from 18 to 109 here and this solar works. It's far less complex and I think more durable than the PV plus heat pump approach. I've used Marathon tanks so they don't even require anode replacement. Anyway, hope that helps. This solar thermal is just different.

Yours, Larry

ps, By the way, I had some fun correspondence with Bill Shurcliff. He was a very clear thinker.

@Jamie Hall The reason passive solar disappeared is because of thermodynamics. To get heat into the structure, the temperature must go up. To get heat out the temperature must drop. No way to engineer your way out of that temperature swing without active means.

Best case scenario passive solar houses ended up the hottest by the evening and cold by the morning. This is exactly the opposite of what most of the population would want. Well designed passive solar did save energy for those that were willing to live with that, the issue is most people would not. That is the reason that super insulation won out in the end, well insulated house created a place that had low energy use but would keep the occupants within the temperature range people like.

For my own home I installed limited passive solar design ideas. Went so far as to optimize the coatings on the south facing windows which reduce yearly energy use by about 2% (which seemed like a steal as it was "free").

The heat would not need to run during most days but it also meant that even on a cold sunny day the house would get up to 80F by the afternoon, many days I was walking around in shorts in the middle of winter. Luckily a manufacturing defect meant I got all the windows replaced, this time around I speced low solar gain and better U value glazing. Everything is better in terms of comfort. The house still get a boost from the sun but no more overheating and cooling loads are way down.

A very light sprinkling of passive solar is a good thing. Too much and you are asking for trouble.

As they say, can't fight thermodynamics.

"My mind is made up and this is what I'm going to do" is, frankly, a rather poor way to learn anything, @BTUser . As @DCContrarian said, why did you bother to come and ask questions at all, or was it just to affirm your ego? We all learn something from each other, and from discussions such as this thread, but if you're not interested, that's your puppy.

Fair enough. Have you calculated your current heating loads? Either through modeling or the process outlined in this article?

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

Do you know your heating and cooling design temperatures, and heating and cooling degree-days?

Have you looked up average monthly insolation for your location?

Without that information you can't even begin to look at a design.

Hi All, I'm probably about to sound silly, but please bear with me. My experience is the most cost effective things you can do to make any heating/cooling system perform is to have a superior shell, so the loads remain small. It's hard, but possible to reduce the loads by 80% with good design, air sealing and insulation. Yup, 80%! If you could get anywhere near that number, any flavor of solar works and is easier to accomplish.

My climate is predominantly heating, but by reducing South and West facing glass, and thinking about low-E glass appropriate for each orientation, I've minimized the problem of overheating. A reason I like to use water as my thermal mass is it can be moved around, but importantly, I don't want to live inside of a solar collector with its temperature swings. I live in a low-mass house.

I have played with a variation of solar cooling, but never completed it because the house works without. The cooling was to have unglazed solar collectors placed higher than a well-insulated storage tank, and filled with an antifreeze mixture. The tank is placed just above the living space and tied into finned tube baseboard elements. Both fluid flows are driven by convection. Thermostatic control for the finned elements is given by a wax filled piston operator, driving a ball valve. It's the same system I use for controlling a gravity driven heating system that I did build, and it works. During the day, the collectors get sorta hot, but do nothing. On a clear night, they radiate heat and convection moves the resulting coolth into the tank.

I got ridiculed when I started to build my place 25 years ago because I used SIP panels, which people didn't seem to understand, and because I used 8" walls, 10" floors, and 12" roof. They told me I was nuts to be that well insulated. I was building for the life of the structure, not current energy prices. Anyway, it doesn't seem so crazy now, and my energy use per square foot is 10% of normal. Hope some of the above is useful.

Yours, Larry

I've heard stories of creative tinkers coming up with passive radiant system and controls. A good place to store some vodka also. Here's to them😎

Hopefully the ROI passes the beancounters math :)

Let me say a word (again) about passive thermal solar or whatever.

The most important word is that trying to retrofit passive solar is… I've never seen it to work well. The house — or other building — has to be designed for it from the ground up, and architected to fit the site and the conditions. No two will be quite alike. There are also some real "gotchas" in the game as well — the most obvious of which is controlling overheating, which can be a real problem in the "shoulder seasons" — mostly because from the solar standpoint the sun angles are the same in the fall — when in most cases you don't need much heat — and in the early spring when, at least in most climates, you most assuredly do. So what keeps you warm in say March is going to keep you really toasty in, say, October.

Storage is not a problem — if it is designed in from the start.

Second, most passive designs which work aren't, really: almost all of them, if not all, depend on some forced air movement if only for adequate air exchange for air quality considerations.

Most are also designed, quite deliberately, to have some warmer spaces (e.g. studies, living areas) and some cooler spaces (e.g. bedrooms). They don't have to, but it makes life a lot easier.

I have real problems with what might be called active solar thermal systems — not that they can't be made to work, but it's not really just a matter of some pipes on the roof. The biggest problem I see with them is heat storage, and transferring heat in and out of the storage in a controlled way.

PV in my view is interesting. If — and only if — you have enough area for a big enough panel array, there is no reason why it can't work — but again, the hitch has to do with storage; only in this case it is obtaining adequate battery capacity and ensuring that that is safe. If you are using PV for heat, you need a LOT of battery storage, unless you are grid connected — and there you may run into some interesting arguments with the rules from the public utilities commission in your State (many of them get difficult if you are trying to generate more power, over the year, than you use, over the year, and there may be limits on the installed capacity).

Hi @DCContrarian , The concept of installing a solar system that doesn't work didn't even occur to me. The basic thought was that with solar, you expect to have less energy to work with, so not leaking it out in various ways is a good thing. And yes, every system needs to be matched to the place it lives… more-so for space heating. I've watched "solar" since the late 1970s, so have seen what was built and how it later got changed to help it work better. I prefer to learn from other people's mistakes, if possible. I both designed and built my place, so site conditions and weather data were taken into account. Designing a low mass house, some sort of storage was essential as I'm off grid, and needed something to even out the diurnal temperature swings. Also, I needed to know how many days of energy storage were needed. Lots of moving pieces. Doing heat loss calcs on a room by room basis allowed me to design the radiant system @hot_rod was so nice to pull up pictures of. Just as an aside, much of the math for it came out of books from the late 1800s and early1900s. I suppose I could have skipped all the work and just wear sweaters by the fireside 😏, but I didn't quite follow Carter's lead. For a while, I got to be my very own lab rat. But the ideas I wanted to test out seem to work, so my nose is less pointy now, and just possibly there are more strategies tested and available for others to use.

Yours, Larry

ps, I did get a bid on doing conventional forced air and my system, (with me doing most of the work) came in at 1/4 the cost.

@BTUser : "One thing that has caught my interest is how little interest has been expressed here is how little interest has been expressed in the potential of solar arrays for providing cooling BTUs."

I had never heard of anyone doing this, I did some googling and couldn't find any examples of it being done.

My quick take is that you're going to be doing a lot of heat exchanging: fluid flows into the collector and exchanges heat with the night sky, then flows into some sort of storage medium and exchanges heat there. Then during the day some sort of fluid flows between the storage medium and the interior, exchanging heat at each end. So at least four exchanges. Temperature delta is the key to heat exchange, it's what makes heat flow, and the overall temperature delta is going to be pretty small, it's basically the difference between the inside temperature and the nighttime outdoor. And that delta has to be split over four exchanges.

The amount of heat transferred is equal to delta times flow rate, so with the small deltas you might need quite substantial flow rates, which might mean sizing the system significantly larger than what is needed for heating. I would also think that a solar panel's ability to radiate heat would be substantially smaller than it's ability to absorb sunlight, because the temperature difference is so much smaller.

Do any ratings exist for panels used for cooling?

My guess is a 3,000 SF house would average about 30,000 BTU/hr cooling load on a hot day. Figure you'd have about 10 hours of cooling, 10 hours when the outside panels are cooling (that's what Stickney figures) and four hours of in-between. So you'd have to move about 300,000 BTU out at night and then absorb them during the day.

A floor that is below the air temperature will absorb about 1 BTU/hr per degree of difference between air and floor. If we assume the same 75F interior temperature and 65F average temperature for the store, that's 10 BTU/hr, so you'd need about 3,000 SF of interior surface — 100% of the interior square footage.

On the exterior, assuming the 15 BTU/sf/hr from my previous post, you'd need 2,000 SF of radiating surface. I seem to recall that the footprint of the house in question is only 1,000 SF so there would have to be some sort of outboard array.

Note that ultimately it comes down to the difference between the interior temperature and the Tplate. You have to assume the same temperature for the heat store on both sides, because it's the same store. So if you make your heat store warmer, which makes the outside radiators more productive, that makes the inside delta smaller and your internal radiators need to be bigger.