Keep Solar Thermal Collectors Cool

Yesterday I was looking at a commercial pool and hot tub for a potential commercial client that wanted a proposal from Simply Efficient and I was reminded of some of the best uses for flat plate solar thermal collectors, and one of my favorite designs.

The potential client has a hot tub that is heated year round and a pool that is heated to 82F May-August and 42F the rest of the year. In my experience, most pools are drained in the winter so this was a surprise. When I heard that they heated the pool at 42F, I thought ‘right-on’. That’s perfect for a flat plate solar system to run at its most efficient at a time of the year when most flat plate solar thermal systems are running at their worst.

Flat plate solar thermal collectors are at their most efficient when the outdoor temperature is close to the temperature of the fluid temperature that is going to the collectors.   Since they ran the pool at 42F, just to keep it from freezing.  I essentially would have the collectors running with 42F fluid to them on 20-40F days.  That’s equivalent to running the collectors in the summer time.   So we get much more output from them than we would when heating fluids up to higher temperatures in the winter.

This worked very well for a client that we installed seven 4’x10′ collectors to heat their snowmelt system.  Generally I don’t see a solar thermal system as compatible with snowmelting systems since snowmelting systems run infrequently and when they do they need a massive amount of heat.  Solar thermal systems operate much better with a constant load.  This snowmleting system was on a condo and they ran it constantly in the winter, regardless of whether or not it was snowing.  They did this to reduce liability from someone slipping on ice.   With this in mind, I tied the solar system directly into the snowmelt system so that I could run 40F fluid back from the snowmelt cement slab and into the collectors.  So this system was running at peak efficiency, even in the winter.

Feel free to comment or ask questions.

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Oil’s Going Up, Again

It was our single minded reliance on oil that originally got me into the  renewable energy industry.  I remember the oil embargo in the 70’s, when we could only get gas for our cars based on the last digit of our license plates.   If you had a license plate ending with an odd number, then you could get gas on odd numbered days.  The lines for gas were really long.  Cars would get in long lines to get gas, and then the gas station would run out before everyone got gas.  There were even fights over cutting in line.  As a young boy, that definitely made an impression on me.

With oil today hovering around $105 a barrel and the political situation in most of the middle east looking very uncertain, it’s hard not to look at the impact that the price of oil has on our economy.   It is not hard to see that we do not have any control over our own economy.

This situation is exactly the reason why I believe alternative energy is more of a security issue than a money issue.  I would rather spend more for energy that keeps money in our own economy than spend it with foreign economies.  Doing so is very sustainable.

When we compare the actual cost of oil.  I mean counting everything (pollution, harm to the environment, transportation….) and also include the heave tax credits that the oil industry receives to offset their risk for drilling.  Then also compare that to the actual long-term cost of different renewable energy technologies.  The renewable technologies will prevail (over the long term).   Least of all, we need to take the money we spend on developing the existing technologies (Nuclear, Oil Shale, Tar Sands…..) and at least match that for renewable energy technologies.  Better yet, we could spend that money directly to spur actual installation of renewable generation capacity.

Currently renewable energy only provides a fraction of our energy mix (I don’t count hydro-electric).  We do not need to worry about destabilizing our electric grid for quite some time.  We need to get moving on installing as much renewable energy as possible and reduce our exposure to middle east instability.

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Light, from a Solar Guy’s perspective

I’ve been watching the solar collectors on my home and also monitoring a commercial system on a dairy.  Today, since were up at a high elevation, we’re above the clouds.  This is rare, but it’s nice and sunny, warm (55F).  Down on the plains its cold and overcast.

Most people are aware that solar systems need sunlight to work.  What many people do not think of is the finer points of sunlight.  In most of the world, a good rule of thumb is to setup the solar system to get the best sun between 9AM and 3PM.   This is when the sunlight is most directly overhead and provides the closest straight line between the sun and the solar collector.  Before 9AM and after 3PM the sun is increasingly slicing through much more atmosphere of the Earth before it reaches the solar collectors.  As the sun goes through the atmosphere it can strike water vapor, dust or other particulates, smog……Each of these strips away a little bit of the suns energy, meaning less for the collector.  Also, if the collectors are fixed in position (non-tracking), which most of them are.  Then more of the sun before 9AM and after 3pm skips off of the glass covering the solar collector as well.

The picture to the right is output from my solar electric system on a good sunny day (8/1/11).  The part that is highlighted yellow is the energy generated between 9AM and 3PM.  Outside of those times there is some, but not a lot of energy generation.  This is why eventhough you may have sunlight from 6AM to 5PM (or more).  Your real energy generation is only during the peak times.

The amount of energy generation also depends on the type of collector and the micro-climate at the location.  For instance, at the commercial dairy installation that we have been monitoring.  They frequently have foggy mornings.  This translates too extremely low output on the mornings.  This is due to two factors.  One, the fog (vapor in the air) is dissipating the sunlight.  Two, the collectors used for this installation are of a concentrating nature (Ten Energistics Suntrac 4×8’s).  Concentrating solar collectors do not work well with the dissipated light (known as ‘diffuse’).  They mostly only operate with ‘direct normal’ light.  This is the light that is perpendicular to the panels.  Which for these panels is whenever they are turned on, since they track the sun to keep their output up.

That’s the basics of sunlight for solar systems.  Please feel free to ask me any questions!

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The Solar Electric Industry is Dead, in Colorado for Now

Today I attended the public utilities comission (PUC) meeting to discuss the solar electric rebates in Colorado.  This meeting comes after the major utility in Colorado handed a sucker punch to the solar industry that is likely to take out a majority of the companies.  The sucker punch came on 2/16/11, when Xcel announced that they are effectively eliminating the solar rewards program.  This program, which was spawned from the voter approved renewable energy standard amendment 37, was responsible for huge growth in the solar electric industry in Colorado.  But now the party is over.

For a moment, the PUC seemed to possibly try to keep an interim rebate going until they can make a decision on whether or not Xcel can do what they are doing.  In the end, they decided to allow the Governor’s Energy Office (GEO) to mediate for the counsel of the biggest parties interested; CoSEIA and a representative for ‘two large solar companies’ as stated by the representative, and Xcel.  They parties will then hopefully come to the next PUC meeting on March 11th with an agreement to move forward.  If not, the PUC can will propose a remedy of their own.

One big question is whether the PUC has any authority to tell Xcel what to do.  Based on my understanding of the rules (I am not pretending to be Counsel by any means) for the amendment, Xcel is sticking to the letter of the law and has reached their quotas.  In fact, they have gone way over their quotas.  They are also spending the maximum 2% allowed to fund the program and have overspent in 2010 and will also overspend in 2011 based on current approved rebates.

Most of this may well be moot since the program is not accepting anymore applications until the issue is resolved.   I doubt that a majority of the smaller programs will be able to wait and will be forced to close.  I wouldn’t be surprised to see a lot of them move to Montana where the solar industry is heating up.

I wish the best to all of us!

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Propylene Glycol in Solar Thermal Systems, Still Not Understood

I was just at a solar thermal workshop for the Heliodyne commercial pump stations.  It was quite good.  What stuck out to me was how Heliodyne recommends pressurizing a solar thermal system to protect the propylene glycol.

First, let me explain.  Propylene Glycol is used in solar thermal systems when they need protection from freezing.  Mostly this is used in closed loop systems (though not exclusively) in parts of the world where the temperature commmonly gets below freezing.  Propylene glycol is related to Ethylene glycol.  Ethylene glycol is what is used in most vehicles to keep the engine from freezing.  Ethylene glycol is highly toxic.  Propylene glycol is considered non-toxic (though I wouldn’t recommend drinking it!).  If you look in your fridge and medicine cabinet, I guarantee that you will find a product with propylene glycol as one of the ingredients.  You will frequently find it in; candy bars, ice creams, shampoos, conditioners, toothpaste (the list goes on and on).

In a solar system the propylene glycol keeps the pipes and collectors from freezing.  However, most propylene glycols break down at a temperature less than the temperature that the solar collectors produce.  This is especially true during the summertime when solar systems produce their most heat and can have their lightest load.  Also known as ‘Stagnation Temperature’.  Stagnation Temperature is the temperature that the collector reaches in full sunlight when the solar system is not operating.  These temperatures can be 320F and higher, depending on the collector type.  The max. recommended temperature of Dowfrost propylene glycol is 250F.  This is important because propylene glycol begins to break down at temperatures above and near 250F.  When the glycol breaks down, it becomes more acidic.  If this happens enough, the propylene glycol can eat through all of the collector and solar system piping!  To extend the life of the propylene glycol, the manufacturers add inhibitors to coat and protect the piping, collectors and heat exchangers.  They also add buffers to increase and maintain the alkalinity of the propylene glycol.

It’s important to know these basics to understand the two trends promoted by manufacturer’s of closed loop solar thermal systems.  They both propose that their way is the best to prolong the life of the propylene glycol.  I’m not certain yet which one is best.

Heliodyne runs their systems at higher pressure’s than all other solar thermal system manufacturer’s.  They do this so that as a solar system heats toward the stagnation temperature, the system pressure rises enough to keep the propylene glycol from boiling.  This way the solar system never boils, so that the propylene glycol does not go through the stress of that phase change and will last longer.

Schott and many other professionals use what is known as ‘steam back’ to protect the glycol.  With steam-back you set up the solar system expansion so that the propylene glycol vaporizes (boils) at a temperature less than 250F.  Since the vaporization pushes all of the glycol out of the collectors, the propylene glycol cannot overheat and is thus protected from breaking down prematurely.

I have been a subscriber to the steam-back method.  However, Heliodyne may have a solution also.  What is needed is for someone to do testing on the modes of breakdown of propylene glycol.  That’s a study I would like to see!

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Thermal Collectors, Use the right tool for the job

Collectors for solar hot water and heating systems are typically the most expensive components in the system.  I am continually annoyed by self-serving manufacturer’s, distributors and installers that want everyone to believe that their panel fits all occasions and all other panels are worthless.

It’s actually quite simple.  Here are a few pointers; a) Different collectors work better for different applications.  b) all collectors work best at lower temperatures for their range.  Mostly, I am writing about flat plate and evacuated tube collectors.  In general flat plate collectors are best for low temperature applications.  Such as; domestic hot water for your home (showers, dishwashing, etc.), low temperature radiant heating.  Evacuated tube collectors are best for High temperature applications.  Such as; commercial water heating (restaurants), high temperature radiant heating.

Next, you need to consider where the collectors will be used.  Is it a cold climate or hot climate?  is the hot water going to be needed year round.  This is where things get a little more interesting.  Evacuated tubes perform better than flat plate collectors in cold climates.  However, evacuated tubes do not melt snow off of themselves very well.  The flat plate collectors operate at their least efficiency in cold climates.  In a nutshell, evacuated tube collectors are best suited to cold climates where you are providing high water temperatures.  If snow is an issue, be sure to tilt up the panels at least to 45 degrees and to not allow snow to build up around the bottom of the collectors.  With the evacuated tubes you will also want to consider whether you need the heat that they generate all year, or only seasonally, like for heating.  Here in Colorado, you need to provide a heat dump to keep the evacuated tube collectors from overheating in the summer, when the heat isn’t needed.

To recap.  Use the right collector for the right application.  This will keep the system more reliable and economical.  Feel free to comment if you have any questions or additions!

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