Determining Solar Collector Flow Rate

This blog entry deals with picking the best water flow rate for the solar collectors in a solar water or spacing heating system.

There are a lot of recommendations around on what the flow rate through water heating solar collectors should be.

These are some of the recommendations floating around out there:

- Heliodyne recommends a range of 0.025 gpm/sf to 0.075 gpm/sf
- SunEarth recommends 0.025 gpm/sf for their collectors
- Solar discussion groups often talk about 0.02 to 0.03 gpm/sf a good number.

So, what is the "best" number, and what are the tradeoffs?

In a nutshell, if you set the flow rate too low, then the collector will run  hotter than it should, and this will result in larger heat losses and a less efficient collector.  On the other hand, setting the flow rate too high results in a larger pump and larger pipes than are really needed, and this increases both the initial and operating cost of the system.

The table below shows the reduction in collector efficiency and heat output as the flow rate is decreased from a high flow level:

All the details on how these flow rates are arrived at here...

Details on sizing a pump to deliver a given system flow rate ...


Gary

Our Prius Goes Over 100K Miles -- Savings Todate $10,400!

Our Prius just ticked over 100K miles a couple days ago.

I've chronicled our experiences with the Prius on this page...

I used the calculator on HybridCars.com to estimate the total savings to date.  I compare the Prius to our other car, which is a Honda Pilot -- its the car we would be driving if we did not have the Prius.  The Pilot is also not too far from the US fleet average.

So, the savings for the first 100K of Prius vs Pilot are:

• 3467 gallons of gasoline
• $10,400 in gasoline costs
• 33 tons of CO2

Truly amazing numbers -- if we keep the car as long as we did our last car, the savings in gasoline costs will probably pay the full initial cost of the Prius!

I know people tend to either love or hate the Prius, but just from an engineering standpoint, its an amazing accomplishment. If we could get the US fleet average just up to what a Prius already gets, we would not have an oil import problem, and it would make a significant dent in our Carbon problem.



Added Sep 4: based on some questions, added this page that compares hybrids to non-hybrids over a 100K miles distance -- including fuel costs.  Also the costs for some big cars/SUVs/trucks.


More stuff on efficient vehicles..   +



Gary

A Nice, Simple PV Panel Ground Mount

This is a nice simple, cheap, and durable way to ground mount PV panels.

The PV panels are mounted to commercially available aluminum rails made for easy mounting of PV panels.   There are two of these rails for each of the sets of 3 panels.  The rails are supplied by companies like IronRidge or Unirac.

The front end of the rails are supported by a 6 by 6 treated timber which runs the full width of the PV array (with one splice).  

The back of the rails that the PV panels are bolted to are supported by vertical aluminum legs, which are in turn supported by a 2nd 6 by 6 treated wood beam that runs along the full width at the back of the array.  The tilt of the panels is set by the length of the vertical legs.

  The two 6 by 6 timbers are in turn supported by concrete piers at about 8 ft intervals.

For the complete description of this mount and more pictures...

We viewed this PV mount at an open house for this very interesting  home by Oasis Hybrid Homes...
 Located in the Paradise Valley south of Livingston, MT --  one of the most beautiful valleys in MT.

Gary

Variation in Riser Flow For Wide Solar Collectors

Solar space heating systems often use wide solar collectors in which a large diameter bottom manifold distributes a heat transfer fluid (often water) to a large number of vertical risers.  The risers are equiped with fins to absorb the sun and transfer the heat into the water flowing in the risers.  The heated fluid is collected by an upper manifold and returned to the storage tank.  The collectors are often arranged so the inlet is on one lower corner and outlet is on the opposite upper corner in order to improve the evenness of water distribution to the risers.

This test attempts to assess how even a distribution of fluid to the risers this design achieves.  If the distribution is uneven, then the efficiency of the collector will suffer.  Even though this design is very common, there is surprisingly little data that I could find on the evenness of the distribution.

The picture shows the my 11 ft wide collector used for the test.

Same collector with glazing removed to show manifolds and risers.

The test was done by removing the glazing from the collector, and with full sun on the collector, and normal fluid flow through the collector, the temperature of each riser was measured with an IR gun style thermometer.
The idea being that if a good fluid distribution is being achieved, the risers should all have a similar temperature profile with the lower part being near the tank temperature, and the temperature gradually increasing as you go up the riser.

Riser No	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
top	124	123	123	124	126	129	124	128	126	132	127	125	129		125	128	126	123	123	122
mid	123	122	121	123	124	124	120	125	123	125	125	125	125		124	123	123	123	123	120
bottom	122	117	119	117	119	117	115	119	118	119	120	121	119		117	117	115	116	120	115
Time	12:47 PM																			12:56 PM
Temp rise	2	6	4	7	7	12	9	9	8	13	7	4	10	0	8	11	11	7	3	7

The truncated table above shows the results -- see the full report link below for the full table -- there are 20 risers in the full table.

Happily, the distribution is pretty even, with most risers showing similar temperature patterns.  There are variations, but probably not enough to seriously effect efficiency.

It does appear that the evenness of the distribution can be effected by relatively small changes in the details of how the plumbing is done.  Have a look at the full report for more details on this.

All the gory details on riser flow distribution test...

If you have additional data on riser flow pattern tests or analysis, please let me know.

Gary

Paul's DIY Solar Water Heating System

This is Paul's version of the "$1K Solar Water Heater".  While the design generally follows the $1K design, he has included several unique features:

- The collector housing (box) is made from metal instead of wood.

- A selective finish is applied to the absorber for better performance.

- A thermally conductive copper adhesive bond absorber fins to riser tubes.

- Fins are formed of heavy gage aluminum using a press.

- Barrel storage tank with PEX custom wound heat exchanger coil.

This is one of Paul's collector fins.  Very nicely formed using tools he built.
He used Permatex Copper Spray a Gasket to adhere and thermally bond the aluminum fin to the copper riser pipe.  This may give better thermal performance -- something I hope to test fairly soon.

The fins were painted by the moderately selective paint called Thermalox  -- this paint is available in easy to apply spray cans and gives a lower emissivity finish that should reduce collector heat loss and improve performance.

This is Paul's collector -- it uses a metal frame made from galvanized steel stud "rail" material.

To store the collected solar heat, Paul uses two barrels, each with a new version of the coil of PEX type of heat exchanger that uses two coils of small diameter PEX pipes in parallel to transfer heat from the solar storage to the potable water.  John Canivan's JC-SolarHomes website has quite a bit of material on using drums for solar heat storage.

Paul  worked out an innovative way to moving the fairly heavy collector up on to his high roof -- probably not OSHA approved, but it worked:

All the details on Paul's DIY Solar Water Heating System...

Other $1K Solar Water Heating System Examples...

More than you ever wanted to know about building a $1K Solar Water Heater...

Gary

Ken's Solar DIY Clothes Drying Rack

Ken was having trouble finding a good clothes drying rack in the store, so he designed and built this very nice solar clothes drying rack.

He provides full construction details, plans, and pictures.  He even included plans for both a deck mounted one and a ground mounted one.

The rack is built entirely from common hardware store materials -- 2 inch conduit for the post, and plastic deck wood for the arms, ...  You should be able to buy all the materials locally.  All the materials are highly weather resistant, so you should get a good life from the rack.

One of the nice features of the rack is that it allows you to place clothes from the washer on hangers, and then hang the hangers on the rack.  The dry clothes can then be moved directly on the hanger to the closet.

 Ken provides dimensioned drawings for both a deck mounted and ground mounted version of the rack.
With the deck mounted version, you never have to step off the deck to hang clothes on the rack.

All the details on Ken's Solar Clothes Drying Rack here...

Some additional clothes drying lines/racks and tons of other good conservation ideas here...

Gary

A Unique Open Flow Solar Pool Heating Collector

Matt has done a really nice job on this easy to build, efficient, and inexpensive pool heating collector. 

This is a unique, open flow collector design which reduces cost while still providing high efficiency.
The collector pump is powered by its own PV panel and heats the pool without using any external power source.  The collector price comes in at about one third of what commercial pool collectors cost.

The picture shows the basics of the design.  Water is pumped from the pool to a header that distributes a trickle of flow to the each valley of the corrugated metal roofing that acts as the collector absorber.  The heat collected by the black metal absorber is collected by water as it flows down the absorber.

At the bottom of the collector, the heated water is collected and routed back to the pool.

Matt's collector is about 64 sqft and does a good job of heatin his 26 ft diameter octagonal  4 ft deep pool in VT.

The total cost of the system was about $260 and the price per sqft of the the collector alone was only about $1.80 per sqft!

One very nice feature of the way Matt did this is the PV powered pump that circulates water to the collector.  By having a separate pump for the collector, the main pool pump (which uses quite a bit of power) does not have to be run for solar collection.  And, by using the PV powered pump, no external power is required for water circulation to the collector.  Powering the pump by PV also means that no controller is required, as the PV panel only generates power for the pump when there is sun on the pool area.

All the details on Matt's low cost, PV powered solar pool heater...

This collector is in part based on this experiment I did a while back on an open flow design.  The experiment includes measuring performance of the collector along with an estimate of efficiency and an explanation for why the collector performs well in spite of some heat loss due to evaporation from the open flow.
It is really nice to see this design working well on a real pool :)

Interestingly enough, this same collector configuration can also be used for cooling.... 
I'm in the process of trying a full scale cooling application using the Solar Shed as described in the link just above.

Gary

Plans for the MA Zero Energy Challenge Winning Homne

The Montague Urban Homestead designed by Doug Stephens was the winner of the Massachusetts Zero Energy Challenge.  The house is a full Zero Net Energy home that generates more power than it uses on a yearly basis.

The 5 KW PV system you see on the roof generates all the power for both space heating (via a mini-split heat pump) and for all the homes electrical loads -- it actually generates substantially more power than the home uses on a yearly basis.

The house design is very effective but also simple and the home is affordable at $180K.

Doug has now made the "as built" plans for the home available as a free download -- thanks very much to Doug for doing this!

Information on the Montague Zero Energy Home ...

Plans for Montague Zero Energy Home ...

Some pictures of the home below.

The double stud, cellulose insulated wall provide cost effective super insulation with no thermal bridging.

Section showing raised heel roof trusses which allow for deep attic insulation.
Double stud walls are also visible.

The concrete slab is fully insulated with rigid extruded polystyrene board.

This small mini-split heat pump provides all the heating for the super-insulated and well sealed home.







The house also includes very efficient windows, solar water heating, south facing glazing for passive solar heating, very efficient appliances and lighting, a Heat Recovery Ventilation system,  and many other energy efficiency measures...

Gary

Half Plan Update for 2010

We started the "Half Plan"  back in 2006.  The initial object was to cut our total energy use in half.  "Total"  was to include energy for space heating, electricity, and transportation -- basically all the energy use we have direct control over.  It seemed like a pretty big commitment at the time.

We went about this in a pretty systematic way.  We worked out how much energy we were using and for what.  Then identified a whole slew of potential projects we could tackle to reduce our energy use, and then went about doing the ones that paid off the best. 

Here are the results in a nutshell:

Space Heating energy use is down from 1610 gallons per year to 700 gallons, or a 57% reduction. 
At usual propane prices, this is worth $1,800 a year.

The CO2 emissions reduction is 12,300 lbs a year.

Electricity use is down from 940 KWH per month to 170 KWH per month, for an 82% reduction.

At 10 cents a KWH, this is worth $925 a year.
The CO2 emissions reduction is 13,900 lbs a year.

Gasoline for car transportation  Switching from a small SUV to a Prius has cut our gasoline use by more than half.
The dollar saving per year at $3 a gallon is worth $1,880 a year.

The CO2 emissions reduction is 11,900 lbs of CO2 a year.

So, we have exceeded the going in objectives in all areas -- sometimes by a wide margin.  In hindsight it was a lot easier than we thought it would be.  No lifestyle changes.  It has also proved to be a very good investment with an excellent return.  

We are still working on further reductions -- I can see lots of remain opportunities.

All the details on the Half Plan are here...

 

I guess the thing that puzzles me most is that there is not more interest in the plan.  It seems like its just a no-brainer way to save energy, save money, and save Carbon.  But, while other areas of Build-It-Solar get ten thousand of visits a day, the Half Plan gets a couple hundred.  I get very few emails on the plan.

Maybe the Half Plan section is poorly done? Poorly organized?  

Does not get the message across?

Anyway, I'd really be interested in hearing your thoughts on why it does not work better and what might be done to improve it  --  you can leave a comment here, or email me...

It seems to me that if this plan were widely adopted it could significantly change the picture of energy consumption and carbon emissions in the US.

Gary

Lee Adds a Self-Propulsion System to His Solar Electric Lawn Mower

In the past Lee has described his project to convert a Troy-Bilt gasoline powered mower to a battery electric mower, and then went on to provide solar charging for the mower batteries.

Lee's latest project is to add electric powered self propulsion to the mower.

First version of the gas mower converted to electric.

The new self propelled mower uses a drive unit salvaged from another scrap Troy-Bilt self propelled mower.

The new self-propelled mower with larger batteries.

All the details on the original mower conversion from gas to electric ...

All the details on the new addition of electrically powered self-propulsion...

Thanks very much to Lee for sending this material in!

Gary