Solar Radiant heat question

todd · September 2010

Greeting,

I need some real world info and answers on a very basic question.

First a bit of background.

I currently have 100 sq ft of flat plate thermal collectors and they work quite well in partial to full sun.
But they don't work very well when in full cloud cover or rain. The problem is I live in the pac nor west (Seattle Wa.) I recently added additional thermal storage so now I need to increase the collector area in order to utilize the increased storage capacity. Instead of adding more fp collectors I am looking at evac tube for the expansion, I plan to keep the fp collectors for now.
The intended use if to heat water for the hot water tank in the house, maintain a hot tub at temp (100) and to provide hot water for radiant heat.

Operating parameters
A typical Seattle cloudy day with an amb. temp of 40F, incoming water temp 80F and a flow rate between 1 and 3 gpm.

The Question:

What sort of water temps might I expect to see if I were to utilize ev collectors?

Since these collectors come in many configurations,
let keep it simple and use a typical 10 & 20 tube array.

thanks

todd · September 2010

Solar Radiant heat question

I have a existing solar thermal system installed and wish to explore the idea of adding under floor radiant heating. Construction is wood joist and plywood sub floor. Area to be heated is approx 600
sq ft.. I currently have 120 sq ft of fp collector area but plan to increase this and add several evac tube style collectors in the near future.

can some one tell me what sort of water flow rates and supply temps are required to heat this area, with an min outside temp of 20 F, average is 40F.

I also like some guidance from some one who knows their stuff, if this project is even realistic given my locations is in cloudy seattle wa area.

russ · September 2010

Re: Solar Radiant heat question

Don't have the numbers at hand but when I studied it for my home in Izmir, Turkey (lot more sun than Seattle) there was no way to make it work.

Your water flow requirement is too high and requires a back up heat source or large storage - with large storage you need ever more panels.

All in all it became very expensive.

Seattle - my birth place! Way back in 1945!

BB. · September 2010

Re: Solar Radiant heat question

Also, don't get too wrapped up with Evacuated Tube type solar thermal collectors...

They are great for very cold weather and very hot water output... But for a price/performance value--standard box with cover glass type thermal collectors may give you more heat overall.

Also, interestingly enough--One of the potential issues with Evac Tube collectors is that they are too well insulated... Snow falls on them, and it takes forever for it to melt off.

With the glass box type, there is enough heat loss that the snow melts off by itself and the collector will gather more BTU's because it is fully exposed to sun while the evacuated tubes are still covered with snow.

Here are couple of things to look at (from another post of mine):

Solar Thermal can be a nice source for space heating and hot water... And usually is "cheaper" per kWhr/BTU vs Solar PV Electric. Also, Solar Thermal lends itself very well to do it yourself projects. Note, these are plumbing projects and have their own issues (leaks, pump failures, installation issues trapping air, anti-freeze, storage, heat exchangers, etc.):

Solar Shed and other Solar Thermal Links

A good place to start reading is Home Power Magazine... They have a free past issue online--and have a lot of articles you can read for free. I don't always agree with them and their reviews--but they are a fun and enlightening read:

Home Power Mag

-Bill

BB. · September 2010

Re: Solar Radiant heat question

Also, Todd, I moved your two threads into just this one... They are similar enough that I think one thread is a better place to address all of the issues.

For evacuated tube, why are you thinking of use them? Do you need >160F water?

Or are you worried about heat loss/freezing?

-Bill

todd · September 2010

Re: Solar Radiant heat question

No worries about moving posts, I'm the new guy here.

So here it is September, noon and my existing collector array is sitting idle with a temp of 80 degrees F.
It's 50 degrees outside and quite cloudy. Being the start of fall, things aren't going to improve any time soon. I need 100 degrees plus at 1 gpm year around to make the system effective.
I was thinking that adding evac tube might be a way to get higher temps during the fall, winter and spring seasons.

Yes I know flat plate collectors are cheap, my last set was $200 ea for 3x6 ft panels, I purchased 6 of them from AAA Solar, used and in very rough condition but still quite usable.
But on a cloudy day they don't do much good, producing barely warm water at not flow rate.
Do I just need to add more flat plate collectors to change that 80 degrees idle temp to something higher or will I just wind up with a larger volume of the same?

I don't recall if I mentioned this, but I have 2 storage tanks, 1 is a 500 gallon hot tub that I want to keep at 101 degrees,, the second is a diy sandbox, containing 50,000 pounds of sand that is buried in the ground, heavily insulated & plumbed with 2100 feet of 1/2 inch pex. It figures that 3 days after I finished the sandbox project, the weather turned, so I haven't really been able to test the performance of the project very well, but initial test show that feeding it 140 degree in yeilds 110 out, so the sand is absorbing and storing thermal energy.

The sandbox will be used in several ways, preheat water going into the house hot water heater, be part of potentially a larger radiant heat system, provide heat to the hot tub by passive heat release , as the base on which the tub will be placed and active heat transfer to the hot tub via direct plumbing means.

I'm also using a Steca TR 0603 controller that does a excellent job controlling the pump and valves.

BB. · September 2010

Re: Solar Radiant heat question

Yea--Sort of the problem with any solar -- No sun, no heat/electricity...

It is not unusual to get only 10-30% output from an array with clouds... With really dark storm clouds, probably even down into the 2% range...

With low solar energy--it does not matter too much how efficient the collector is--you just are not going to get much heat.

Sort of gets back to the first rule we follow around here... Conservation--it is a lot less expensive, gives you a better return on investment, and can make your home a lot more livable (less drafts/hot/cold spaces, less noise from outside, less dust, etc.). Plus cutting your "wasted energy" from old inefficient appliances, computers, etc.--You don't have to pay to move the heat outside (if you have A/C).

-Bill

todd · September 2010

Re: Solar Radiant heat question

BB. wrote: »

Also, Todd, I moved your two threads into just this one... They are similar enough that I think one thread is a better place to address all of the issues.

For evacuated tube, why are you thinking of use them? Do you need >160F water?

Or are you worried about heat loss/freezing?

-Bill

Thanks for the reading I already subscribe to Home Power, It's completely at fault for my conversion to solar thermal, If it weren't for the publication, I would have just gotten rid of the hot tub.
and dismissed solar energy projects. Now a year later I desire to expand the system to provide more heating solutions. So many friends and neighbors have commented does it actually work?

I also think that I need to add a evac tube collector to provide the higher water temps needed during the cooler months of the year. Snow really isn't a concern in my area, I can be creative on how they are installed. I'm told that near vertical 70 degrees or so, and a light applications of silicone spray will virtually eliminate snow accumulation, and the evac tubs will better collect solar energy when the sun is lower in elevation.

One other thing I failed to mention, I need about 10,000 btu to heat the sandbox, 1 degree.
Since pex has a temp limit of around 180 F, I need somewhere around 600,000 btu to heat it to 140.
Heat loss appears to be 1 to 2 degrees per 24 hour period. I'll have a better idea once I access the memory card from the controller.

todd · September 2010

Re: Solar Radiant heat question

BB. wrote: »

Yea--Sort of the problem with any solar -- No sun, no heat/electricity...

It is not unusual to get only 10-30% output from an array with clouds... With really dark storm clouds, probably even down into the 2% range...

With low solar energy--it does not matter too much how efficient the collector is--you just are not going to get much heat.

Sort of gets back to the first rule we follow around here... Conservation--it is a lot less expensive, gives you a better return on investment, and can make your home a lot more livable (less drafts/hot/cold spaces, less noise from outside, less dust, etc.). Plus cutting your "wasted energy" from old inefficient appliances, computers, etc.--You don't have to pay to move the heat outside (if you have A/C).

-Bill

Don't get me started on A/C, In some ways Seattle was lucky this summer, I had 2 weeks where I had to run the a/c, my power consumption jumped from 30 kw per day to 45-50.

So are you saying that no matter how many collectors or their type I add I won't be able to get a usable water temp for the 6 months of cloud cover Seattle gets?
Since I have 80 degrees now I find it hard to believe getting an additional 20 or more degrees would be a unreachable goal, even at very low flow rates. I run the grundfos pump between 60 and 100 %, but I can go lower, down to 30% or 40 %, as I recall.
Granted there are some better days than others and last Fall the array and pump were running, just not at a high rate, this is in part why I think that maybe adding a 20 or 30 tube evac to the system might help boost the temp on the lesser cloudy days to trigger the system to operate.

BB. · September 2010

Re: Solar Radiant heat question

It has been decades since I had to do my Thermal calculations from college--So I am not going to say "never"... But at some point you probably will hit the laws of dimisishing returns... If you need 4-5x the amount of solar thermal collectors to get an extra 100 heating days a year (all made up numbers)--then it becomes a real $$$ of installation costs (and added maintenance) vs the avoid fuel costs.

I grew up on the coast just south of San Francisco CA... Lots of trees and marine layer (my little valley, 4 days of heavy overcast, 2 days of sun during the "summer"... Back during the 1970's oil shocks--There were people coming in to sell solar hot water collectors even in our area... Never heard of any successes and most were gone within a couple years (these were simple batch heaters from what I remember).

Or to quote somebody who can write in English:

"The coldest winter I ever saw was the summer I spent in San Francisco."
--probably not written by Mark Twain

Since you have an existing system--You can run your own experiments with a flow meter and a couple thermometers...

Take a day you are interested in and run several flow (low to high) rates and document the temperature increase from inlet to outlet... You should be able to graph a curve that shows how many BTU/kWH per square foot of panel you are currently getting... And lay that against the vendor's performance curves...

Obviously, the slower you move the water, the higher the temperature--but since water flow is so slow, you get very little heating effect. That is the point at which heat loss (from a hot panel) equals heat gain (from a weak sun)... And see if you can match that against the performance curves of other collectors.

-Bill

todd · September 2010

Re: Solar Radiant heat question

Sounds like an interesting experiment, I'm not sure if I can force the controller to run at a particular flow rate, I think it only has 3 speeds off, on and automatic. I'm also not sure if the results would be linear.
The controller displays calculated btu and kw outputs for a particular temp and flow rate so that part is easy.
But I'll see what I can do. I've seen some of these graphs and being novice level at this technology, I'm not exactly sure how to read them or translate them into useful info. I could contact a supplier of equipment and ask but I suspect they will say just about anything to make a sale, in light of the current economic situation.

What I am hoping to find is a home owner in my area and get actual performance info for one of our several cloudy days.

BB. · September 2010

Re: Solar Radiant heat question

You might be able to play with the system--Get a liquid flow meter (such as from an old swimming pool setup) and plumb in a valve to restrict flow to a particular value--then measure/plot the results...

And yes, it will probably not be a linear graph--You will probably see a fixed BTU per hour high very high flow rates and the BTU/Hr value falls as flow rates are reduced and the temperature approaches the stagnant temperature (flattens to zero BTU per hour) of the collector.

It would be interesting to see what you find out... There are so many competing factors (temperature rise, convection, conduction, radiation, etc.) all of which have their linear, squared, 5th power etc. effects your installation.

And you are probably correct--you run the risk they will tell you want they think you want to hear.

The technology is probably "cheap enough" that you could purchase a few tubes (used?) and a flat plate collector and run your own tests.

-Bill

todd · September 2010

Re: Solar Radiant heat question

Yah, that will work and already installed.

So I want to record, the temps for inlet, outlet and flow rates?

Then what? I may be able to view some graphs from manufact.. companyies but what do I want to look for?

And how will this information answer my original question?

LucMan · September 2010

Re: Solar Radiant heat question

There is no simple answer to your question.
There are a lot more calculations that you will have to make to get to the answer that you are looking for.
Start with a heat loss calculation for the area that you want to heat.
Based on the results you can calculate how many MBTU's you will need for an entire heating season in Washington.
Then the SRCC web site can be used to determine the output of EV tubes vs flate plates under cloudy conditions. EV tubes have a much higher output under partially cloudy conditions (2K vs 10K BTU per day for 32 sq') than flat plates as long as the temperature stays above the point where the EV tubes could frost over.
PV watts will then specify how many BTU's are available to you in your climate zone.
When you get done with all that you will realize the enormous amount of tubes and storage that you will require to heat your house with 100% solar thermal. Then there is the question of what to do with all the excess BTU's during the summer. Possibly heat a pool?
My suggestion is to shoot for about 50% of the total required, and use some sort of backup system for the rest.
Probably a high efficiency heat pump system would fit well for your climate. Look into Daikin Altherma air to water.
Flow rates are not as critical as total BTU output. And adding 1 30 tube EV will only give you 10K btu per day more under cloudy conditions.

todd · September 2010

Re: Solar Radiant heat question

Are you sure there isn't a simple answer?
I've given the inlet water temp, the flow rate, the geography and the weather.
Someone must have a system installed that can illuminate the question.

Heat loss that's for a structure and they are what they are. It's a typical 1400 sq ft manuf. home build in the early 1980's,

BUt heat loss calculations aren't important since I am trying to heat water not a building.

I'm trying to determine the temp rise a evac tube collector will provide, or if adding a more fp collectors will provide the gains I'm looking for or just have a larger volume of water stored in the array.

Consider this, If 100 sq ft of collector and of typical design with single pane glass will heat to 80 for a cloudy day, what water temp might I see if I were to increase the area to say 150 or 200 sq ft?
Or would adding a evac tube be a more effective means to achieving a desired temp goal?

All I can say with certainty is with the addition of more thermal storage, I need to add more collector area.
While I don't mind experimenting, adding in small quantities at a time, I need to find out if evac tube would be the better choice

Because of having so many heating sources, I don't expect any one to fully provide the solution to house heating.
I currently use a fireplace that provided 99% of last winters heating, it basically eliminated the need for the forced gas furnace, this past summer saw the addition of a electric under tile floor radiant heat strip, about 1800 watts.

I already have the enormous storage area, a sandbox holding 50,000 pounds, it measures 4x7x9 ft.
Calculations indicate it will require something on the order of 600,000 btu to heat to 140 degrees, not including heat losses which should be minimal being a solid it's upper temp is limited by the plumbing material and the boiling point of water. I purposely build the sandbox large to act as a thermal dump device.

BB. · September 2010

Re: Solar Radiant heat question

Todd,

I hope that you find the following helpful... Sometimes I type the answer to a question that was never asked--So be kind... I am trying (my wife says I am trying too).

Depending on what your flow meter is calibrated in (I am thinking of something like one of these--you have to find ones that cover your expected flow range). Or, for a short experiment you can even use a "calibrated" 5 gallon bucket (if you are using plain water) and dump 5-10 gallons into the buck in X minutes...

So, lets assume that you do from from 0.1 gallon to 10 gallons per minute flow rate.

1 gallon of water weighs ~8.35 lbs. So 10 gallons of water weighs 83.5 lbs (per 10 gallons).

And you will be doing these measurements in degrees F (Fahrenheit). And BTU (British Thermal Unit) is the amount of energy it takes to raise 1 lb of water 1 degree F.

So, lets say you start on a cloudy day (say high overcast in the middle of the day so that you can run several tests with similar weather conditions) with 10 gpm (gallons per minute) flow on your 120 sqft solar collector... So you set your flow rate to maximum and find out that it fills your bucket with 10 gallons of water per minute. And you find out that the temperature increases 1 degree F... How much heat did you collect... By the way, if you want you can convert this to kWatt*Hours (1kWH equals 3413 BTU) (note, obviously these numbers are made up--but I did size them very roughly for what I would expect your solar thermal array to produce in cloudy weather).

I don't know your controller and what exactly displays--but here is an example of how you would manually log the data for three differing operating points.

10 gpm * 8.35 lbs per gallon * 60 min per hour * 1.0 F temperature rise = 5,010 BTU per Hour
5,010 BTU per hour * 1/3,413 BTU per kWH = 1.47 kWatt heating rate

Now, lets do 1.0 GPM and assume you get an 8F rise:

1 gpm * 8.35 lbs per gallon * 60 min per hour * 8.0 F temperature rise = 4,008 BTU per Hour
4,008 BTU per hour * 1/3,413 BTU per kWH = 1.17 kWatt heating rate

And then, the same thing at 0.1 gpm and assume you get a 40 F temperature increase:

0.1 gpm * 8.35 lbs per gallon * 60 min per hour * 40 F temperature rise = 2,004 BTU per Hour
2,004 BTU per hour * 1/3,413 BTU per kWH = 0.59 kWatt heating rate

So, you now have a few data points that show you how much heat you get on a cloudy day... Do the same thing on a clear day, and at a couple different temperatures (hot day vs cold day)...

With your system, I guess it does recirculate water, so you have the ability to observe system performance with different inlet water temperatures... A set of logs may include the following:

Temperature Ambient / Input / Output / Low-Med-High speed / BTU.Hr or kWH output

40F / 60F / 61F / High / 1.0 kW
40F / 60F / 64F / Med / 0.8 kW
40F / 60F / 68F / Low / 0.3 kW

Then wait for the weather to change to a hotter day (say 60F) and run the same tests...

The problem is that you don't have a thermal reference point for how much sun light you are getting... You can get a pyrometer to measure actual heat content from the sun, or you can use a simple solar cell/small panel and a DMM set to Amps (or mAmps--depending on size of panel).

Build your own solar irradiance meter

Within a reasonable level of accuracy, the short circuit current from a solar cell represents the amount of solar energy you are getting (at least in the light/UV range of the silicon cell).

From your location, you would assume 0.0 as "dark" and perhaps 80.0 mAmps on a sunny/clear summer day. If you get 20.0 mAmps on a cloudy day, then your solar irradiation is 20/80 or 25%. Typically a bright/sunny day is around 800-1,000 kW per square meter (you may have the solar cell/panel information and it will tell you the output current at 1,000 Watts per sqmeter--High Noon sun).

And you can compare how much useful heat you get from your 25% day and figure out how many more sq.ft. of panels you wish to add... And if your system outputs very little Watts (or BTU/Hr) on a cold/overcast day--then 3x a very little amount of energy is still a little amount of energy.

Then you can look at the performance specifications for an evacuated tube
module and see if your setup (out door temperature, flow rate, output temperature) and figure out what a 10 tube array would output in similar conditions.

So, now that I went into way to much detail about how to understand the performance of your current system--Here is a set of charts from a evacuated tube type collector manufacturer:

www.thermomax.com/Efficiency.php

So, on the last page, if you want a 40F rise in temperature on a 0C day (32F) on a 200 W/sqmtr (roughly 20% sunlight) then the efficiency would be (very roughly--goes off the end of the chart) 20%...

So, for 120 sqft of evacuated tube collectors is 11.1 sq meters:

200 w/sqmtr * 11.1 sq meter * 0.20 eff = 444 Watts output

Is that a lot or a little compared to your existing setup and is 444 Watts worth the $$$ to purchase install?

I don't know--that is your call....

Anyway, that is how I would perform an experiment on my system (if I had one) and wanted to look at how to collect more heat.

I hope that helps, and I did not confuse you or myself even more.

-Bill

System2 · September 2010

Re: Solar Radiant heat question

Todd:

I live in northern AZ. and have run a 32 tube system for almost 2 years to the day. Although we do not have many cloudy days here, the few that we do get we will still on average run at about 120-125 degrees for the day.

I should clarify that this is during the summer when all we are using it for is to heat the domestic hot water.

During the winter when we are heating both the radiant floor system as well as the hot water tank we probably are averaging best guess about 90 degrees on a cloudy day at about 35 degrees outside temperature.

heynow999 · September 2010

Re: Solar Radiant heat question

A Resol BS4 controller has variable speed ability. It will start out at %30 pump speed and then speed up in small increments if the DT continues to rise. Very smart. Make sure you get the BS4, it is only a few dollars more. It can also controll two pumps if you want to dump heat, etc

BilljustBill · September 2010

Re: Solar Radiant heat question

todd wrote: »

Yah, that will work and already installed.

So I want to record, the temps for inlet, outlet and flow rates?

Then what? I may be able to view some graphs from manufact.. companyies but what do I want to look for?

And how will this information answer my original question?

In your record keeping, because this is thermal Heat, wouldn't you need to include the outside temp within each of your season changes? Depending how the collector is insulated, the outside temps affect it's production numbers.

To keep it standard for future data comparison, one could use the "offical" start of winter, or spring, or summer, or fall as standard boundries for your data....

Just a thought,
Bill

Solar Radiant heat question

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