Solar Integration

Mike T., Swampeast MO

Know many consider it impractical to integrate solar with space heating and to instead use for DHW production.

Really want to try something with a condensing modulating boiler (Vitodens) however. It's connected to a system with proportional flow via TRVs and some small "wild" radiant floor loops. Goals:

1) Prevent as much of the "pulsing" operation of the Vitodens as possible.

2) Use the solar component for space heating when the supply requirements are VERY low--like 85° or so with a meager delta-t near 5°.

------------------------------------------------

Around 40°, my house starts loosing less heat than the minimum possible via the Vitodens 6-24 output (approximately 21 mbh).

Have been playing with "sun" and "moon" dial settings as well as external switching of the heating program.

For over a week have been switching to the reduced (moon dial) setting with temps above 40°. Moon dial set at 57°. 0.8 reset slope with 14°C (25°F) shift so at 58° outdoors boiler target is about 83°. At 40° outdoors it's about 96°.

Yes, I'm using the "sun" dial at night and the "moon" dial on warmer days.

Actual temp in the supply mains in this outdoor temp range is from about 74° - 82°.

Despite the low temps in the system, there is no objectionable drop in indoor space temp. (The radiant floors do however get quite cool--staying only 6-7° above the air temp.)

Clocking the meter and estimate based on flow/delta-t both agree that heat being given off by the system is 4-5 mbh or lower.

Have a piping arrangement worked out that will function with the Vitodens with the only Vitodens input whether or not the burner is firing.

My logic is that as long as I can keep the supply temp at least 85° the burner will not fire until the outside temp falls below about 40° and the unit goes into the normal "sun" mode setting.

Collecting about 7 mbh during ideal conditions should be great. Heat from solar transfers in a dual-coil tank via the lower HX coil. Water for space heating is that in the tank itself; upper coil for supplying DHW. Should be able to operate the solar loop circulator for the most efficient heat transfer in the collector. Flow will be low, collector input low temp; collector output high temp. Delta-t across the HX coil in the tank will be high as well. Aren't those good conditions for efficient collection?

Excuse my stupid question, but what kind of collector area do I need?

These should be the conditions to be met--let me know if you need anything else. If too much, can you give a good reference, web, book, etc?

1) Latitude: 37° N

2) Collector angle: 38.5° (know that's not ideal, but that's my roof angle). Possible to mount collectors on the ground, but would have to get permission from the University who owns the adjacent vacant lot (unusable unless they buy and demolish my home).

3) Collector orientation: about 3° east of due south.

4) Collector type: Plain preferred but evacuated tube possible.

5) Desired ideal peak collection: 7,000 btu/hr @ 70° outdoor ambient as measured by the collector (e.g. solar influenced outdoor temp).

6) Presumed temp of water entering collector during space temperature mode: <85° (Higher temps will mean that an excess amount of heat is being collected.)

7) Presumed temp of water leaving collector during space temperature mode: Unknown. Can be adjusted via flow for ideal of???

Mike T., Swampeast MO

Here's a link for daily local available solar radiation from February 1, 2005 through current.

"Delta MO Solar Availability"

hr

Storage is key

here are some solar classes that are coming up

www.usasolar.net

www.solarenergy.org

Maybe get Dale P, Dave S, or Mike T to help you design a system.

I expect to be taking a bunch more panels off this spring, let me know if used panels interest you. They sell quick around here!

hot rod

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mike T., Swampeast MO

"Storage is key"

Sort of understand that, but am thinking unconventionally--as usual.

Maybe 7 mbh or so peak collection is high for solar, but it doesn't sound like too much...

Sure seems that I could collect solar very efficiently with such a low entering temp and ideally use it at very near the rate it's being produced.

The solar will only be used for space heating in a narrow band of outdoor temps--say from about 40° - 55°. On a really good day, I should be able to collect enough energy to keep the solar providing heat through the evening until the temp falls below 40°.

During warmer or cooler weather, the solar is collected for DHW.

Attached is an output graph from the Vitodens with the new sun/moon settings and changeover. Supply target does NOT reflect the moon setting--only the point of WWSD. Will add the ability to my program, but don't have time now.

Remember: the outside temp is SOLAR INFLUENCED. If you study, I think you'll see how this system is possible.

hr

What will you do

with the output from that much collector if you don't have a heat load? See, if you had some storage capacity you could dump all that summer solar output into a tank and sell DHW to the school next door

hot rod

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mike T., Swampeast MO

It's going to a big indirect DHW tank in warmer or cooler weather. Everything via the one circulator built in to the Vitodens.

Only thing not fully considered is overheat. But, doesn't collector efficiency make a drastic drop when the incoming temp gets high? Lots of versatility in the Vitodens DHW production so should be able to able to come up with a good and efficient control scheme.

Mike T., Swampeast MO

Am not trying to re-invent the wheel. Do understand that a lot of solar wisdom was gained during the 70s.

But, was anyone back then concerned with trying to provide the "bottom end" of a modulating boiler supplying a proportional flow system during only coolish weather when supply temp requirements are extremely low AND you're also receiving natural solar gain in the structure?

Larry Weingarten

random thoughts

Mike: Since you're looking for low end solar temps, try using a relatively inefficient collector. Stick with flat plate single glazed with black paint rather than black chrome. Avoid evacuated tube. You want the efficiency to drop off as things heat up to avoid overheating. I've got six four by eight foot collectors on a 1000 gallon tank and they heat it up to 100* in two sunny days. That's over five gallons per square foot and it still gives me something usable for space heating. The coil arrangement I've used in the tank is just what you describe except there is another loop at the top of the tank for space heating water. It's a non-pressurized tank. 80* water keeps the house at 70*. There is essentially no DHW load yet, so things will change some. I'd size the collectors to the load and size storage generously from collector surface area. Do you have any old SRCC data books? (I'd guess you can expect 600 to 750 BTUs/square foot/day. They would help you figure output from collectors. Experience speaking here... use high temp insulation on solar panel piping or expect melted insulation... particularly if you're considering high delta T! Done rambling. Hope it helps.

Yours, Larry

Bob Gagnon plumbing and heating

more random thoughts

I have a solar heating system that has been working good all winter. I like the evac. tubes as they provide a lot of heat in the cold weather. I think a 65 to 70 degree angle is best for heating. IT is aimed more to the fall and spring sun, it is pointed away from the summer sun when you only need domestic. You could simply cover some of the tubes in the summer. The steep angle keeps snow off the tubes and may provide protection against hail. A large tank is important, Hot Rod showed me how to make a large 1500 gal. tank inexpensivly with roll rubber roofing. After using my system for about 6 months I am more optimistic than ever. The domestic is nice but the heating is a bigger piece of the pie. Go for it! Bob Gagnon

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mike T., Swampeast MO

Thanks Larry!

Definitely helps. Glad for the rambles.

Since collectors will be oversized for the domestic load (with a much higher heat requirement) I want collectors that drop their efficiency in less than ideal conditions. Right? (At least the oversized part can be inexpensive...)

It's the "generous storage" that still confuses me.

Say the primary tank is 60 gallons and well-insulated.

Say at 8:00 p.m. the temp in the tank drops below about 78° and the draw from the tank stops. Overnight it will probably drop to about 65° or so.

I then need to heat the water to about 82° before it can be used, so I need to collect 8,500 solar BTUs. Say I can collect 63 btu/hr/sq.ft of collection area and I want to collect 6,000 btu/hr peak. So I have about 95 sq.ft. of collector area. It's the morning, so let's say I average 60% of the ideal during heat-up or about 3,600 btu/hr. It will then take about 2½ hours to heat the water in the tank to the minimum useful temp.

So, the tank should be ready for use at around 9:00 - 10:00 in the morning--just about the time on a "typical" shoulder day that the boiler is shutting down from its overnight burst. At the end of those bursts, the boiler has fired to a temp significantly over its target, so there'e already some excess heat available in the 300 or so gallons of water in the system.

The outdoor temp goes above 40° and the Vitodens is switched to reduced temp (moon dial) mode with the circulator operating at a fixed capacity of about 30%. It's a nice sunny day and I switch the outdoor temp sensor of the Vitodens over to one slightly influenced by the sun--this serves to lower the boiler target temp to 80s.

System supply water begins to divert through the solar collection tank. Draw from the tank should be 4,500 btu/hr or less. As long as the water leaving the tank is warmer than the water entering, diversion is allowed. A modulating 3-way mixing valve allows the diversion--the greater the difference between the water entering the tank and the water in the tank, the less it opens. The closer the water entering to the temp in the tank the more it opens.

As long as the supply to the system stays above 82° or so, the Vitodens shouldn't be triggered into giving a "pulse" because boiler temp has not fallen below its threshhold. (BTW that threshhold seems to be a BIG part of the operational data store as it gives every indication of having changed over time in this system.)

Even if the water leaving the tank (system supply) rises significantly, the TRVs will do their job, overall flow will reduce and BTU output will stay similar.

Sun is shining nicely and the collectors are operating at peak output of about 6,000 btu/hr. Despite supplying the space heating load, the water in the tank continues to rise at about 3° per hour.

Now, the outdoor temp (solar influenced) exceeds about 60° and the Vitodens goes into shut-down. This typically occurs between noon and 1:00 p.m. Tank temp should be around 86° or so. For the next 2-3 hours, all of the panel output accumulates in the tank with a rise of about 12° per hour. Tank temp should be about 110° - 122°.

Outside temp (solar influenced) drops below about 60° in mid- to late-afternoon. The circulator resumes operation and the solar tank provides the heat. Solar collection has passed its peak, so collection should be very near use and the tank will stay nearly constant in temperature.

So, when the sun goes down I have 60 gallons of water at about 115° to use. The Vitodens outdoor sensor switches back to the North, but is still above 40° so it remains in reduced temp mode. It typically stays there until at least 8:00 p.m. or so.

I have 17,500 BTUs available in the tank before it's depleted. Those BTUs will be used at a rate of 4,500 btu/hr or less. Say solar collection effectively ended at 4:30 p.m. I have about 3.8 hours of solar heat available so the tank is depleted around 8:30 p.m.

Shortly after, outdoor temp falls below 40°, the boiler goes into normal (sun dial) mode and modulates throughout the night into the next morning when the solar again accumulates.

---------------------------------------

Realize I'm talking in ideals, but in reality there should be at least 30 days of such weather each season with decent sunlight. I haven't saved a huge amount of gas (maybe 20 therms NOT including DHW production in warmer/cooler weather), and I have stopped thousands of "pulses" from the Vitodens--certainly can't hurt the life of the equipment...

Larry Weingarten

Part of...

what makes your solar "fun" is you're designing to a different test; filling in around the boiler. But if some of the assumptions are off only 10%, the system might not perform too well. Increasing storage from what you have now,(roughly 2/3 gallon per square foot of collector) to, say, two 80 gallon tanks piped together top and bottom, (approx 1.7 gal/sq-ft) would give you a bigger flywheel, evening out the unknowns. Bob's right about using evacuated tube if the tubes are tilted up higher, say latitude plus up to 20 degrees to help prevent summer overheating.

I just get a little squeamish when solar is designed so closely. Dust on the collectors, loose insulation, sensor drift, debris in the pump impeller... all throw nice calculations out the door.

I'll add, your modulating three way mixer for drawing off the solar is elegant!

Yours, Larry

Mike T., Swampeast MO

Don't know if you've seen before, but here's the piping diagram. I played with a standard Viessmann design so some of the components listed may not be used. The "meat" is pretty good I think.

Don't have the DHW feature labeled, but it's really simple. If temp in solar tank exceeds the temp of water coming OUT of the DHW tank during a call for heat, the water flows through the upper HX coil in the tank. Simple (non-modulating) 3-way valve handles the diversion. The Vitodens will continue to bypass water to heat the DHW tank, but since the incoming water will be artificially high, the burner will either modulate down or even shut down completely while the boiler "thinks" it's making DHW.

Lots of versatility in the Vitodens DHW production (including some changes that can be made via external inputs) so believe I can allow it to produce significantly higher water temp in the DHW tank when solar energy is abundant--but not so high when unavailable. Tempering valve on the DHW tank of course

Understand what you're saying about "cutting things close", but if I add significantly to the storage capacity, won't that greatly increase the time it takes to make low-temp heat available to the space heating system?

If it takes too long, the burner will still "pulse" during the day. Will have more heat available for night, but once the boiler switches to the normal (sun dial) setting and starts modulating, I loose the ability to inject heat into the system without some sort of unavailable integration to the "brain". Also, the supply temp requirement will be significantly higher so most of those BTUs would just sit there overnight slowly giving off their energy to the basement where I don't need it. Guess it would "catch up" IF the next day is nice and sunny...

See where you're coming from with the "unexpected" variables that can throw off the calculations, but when that happens, the Vitodens will just resume its "normal" pulsing and I'll be immediately aware that something is wrong via the datalogger.

Gotta' dig and hit the books, numbers and historic solar radiation availabilty. That MO Agricultural site has hourly data available for a number of years and less than 10 miles away.

siggy

active solar systems

It's interesting how much interest there is in solar energy collection now that fuel prices have escalated.

Our office has had so many inquiries on solar that I've incorporated a new section on interfacing solar with conventional hydronic systems into the upcoming Modern Engineering Concepts for Hydronic Heating Design seminar series.

In a nutshell I vote for flat plate collectors in a gravity drainback piping design. It's simple, efficient, and reliable. You can interface it with either a conventional boiler or condensing boiler. We'll be getting into the nitty gritty of design during the seminar.

If interested in the seminar check out www.bnpmedia.com/events.htm

Siggy

siggy

interest in solar

It's interesting how much interest there is in solar energy collection now that fuel prices have escalated.

Our office has had so many inquiries on solar that I've incorporated a new section on interfacing solar with conventional hydronic systems into the upcoming Modern Engineering Concepts for Hydronic Heating Design seminar series.

In a nutshell I vote for flat plate collectors in a gravity drainback piping design. It's simple, efficient, and reliable. You can interface it with either a conventional boiler or condensing boiler. We'll be getting into the nitty gritty of design during the seminar.

If interested in the seminar check out www.bnpmedia.com/events.htm

Siggy

Mike T., Swampeast MO

Am curious Siggy.

Do you think my scheme has a reasonable chance of working?

Played with solar as a teenager but never had much chance afterwards.

Larry Weingarten

I've been studying...

...your diagram and have a question. For solar preheat on the DHW, it looks like the solar is competing with the boiler at delivery temperature. If it were possible to run your fresh supply through a coil in the solar tank before entering the DHW tank, wouldn't that extract all those low temperature BTUs that seem to be missed now? If another coil is not possible, it might be interesting to compare to see which serves you better in the long run, preheat coil or as you have it drawn.

A completely different point about the volume of storage... If the insulation is good, (I put 5" foam around my tank) your overnight loss will not seriously deplete the tank. If it takes the solar two days to charge the system instead of one, well no problem. You'll just use those BTUs the day following when they were collected. I put to you that sizing the collectors to match the load along with a little for heat loss throughout the system might not be bad ;~)

Yours, Larry

Mike T., Swampeast MO

Couple reasons (maybe both are bad).

Seemed a simple way to integrate into the Vitodens without special, expensive controls.

Since solar collectors will likely be oversized for typical DHW loads, reasoned that the DHW tank could become a sort of "dump" for excess heat that might build up during the day when there's little draw.

Thanks for your patience with a solar dim-wit. Will do some serious study in the next few months and get back with you.