CAD Model of Planned Solar Hot Water System

   Initially I thought I had the ideal spot for the solar hot water panels.  After a little bit of CAD work in PRO-E, I found that the spot was not ideal, but usable.

    Size matters:  Solar radiation falls on the earth at 1366 Watts/meter² at midday in the summer.  This is slightly less than the power required by a coffee maker.  But we don't wish to heat a small pot of coffee, we want to heat something like 80 gallons of water to 180°F.  So we will need more than one square meter.  Also, the panels are not 100% efficient, but that can be the subject of another post.

     It is fairly common to use two panels that are 4×10 feet in size for domestic hot water, or about 80 ft² or 7.4 m².  For the location that I had chosen, this would not work because it would block some first floor windows.  So I decided, instead, to use three panels that are 4×6 feet in size as shown in the image.  This is 72 ft² or 6.7 m², so just a little bit less the area of the two 4×10 panels.

   The location is just outside of my dining room bay window and represented a few challenges.  Firstly, I didn't want the panels to block any of the dining room window for obvious reasons.  That limited the height of the assembly.  Secondly, these panels will need to shed snow in the winter, so the bottom should not be too close to the ground.  In fact, ideally the bottom would be at least 3 feet off the ground so that snow sliding off the panels wouldn't form a pile at the base and block the panels.  In the end, I decided to compromise on the snow issue.  I will simply have to shovel the bottom of the panels out in the winter.  In any case, it should be clear that the 4×10 panels would not work here as they would block the window.

  The location is on the back of the house and has a concrete walkway right at the location where the panels need to go.  The walkway is, surprisingly, not parallel with the house.  So in the left side of the image you can see that the concrete pier I am planning barely touches the walkway.  However, on the right side of the image, the concrete pier is almost in the middle of the walkway. 

  Part of the walkway will need to be removed and recast.  Annoying.

   The grade is sloped from the left to the right.  So the piers on the left will stick out of the ground a lot less than the piers on the right.  Not really a problem, but aesthetically not very pleasing.  I think the concrete piers will be ugly.  It would be nice to think of a way to hide them from view.

    To help with the appear, I decided to center the three panels on the center of the dining room window.  This leaves a little bit of space to the left, perhaps I can have some flowers planed there.  The area to the right needs to be left open so that the oil-guy can reach the oil filler cap.  Nice that the solar panels can hide some of the fossil fuel hardware which is stained with oil and frankly a little smelly.

  The location faces almost perfectly south and has great solar coverage from morning until about 3pm for the summer.  Winter coverage is going to be poor due to the location of trees and the low elevation of the installation.  The panel angle will be fixed at 45 degrees which is roughly the latitude up here in the Boston suburbs.  This is generally considered a good compromise between optimum summer and winter heat collection.

    Next post, I will talk about the solar coverage measurements.

AC or DC Powered

Most solar hot water systems require a pump and a controller, both of which require electricity.  That electricity has to come from somewhere, and there are two choices. 1) plug into a wall outlet, so-called AC power, or 2) use a PV solar panel, so-called DC power.  Most people with access to the electric grid use AC power whereas people that live "off-grid" choose DC power because they have to.  For my project, I could use either AC power or DC power, they both have their advantages and disadvantages


AC Power:  This is the simplest, least expensive, most flexible way to power the equipment needed to run solar hot water panels.  Here in New England, most people use hydronic heating, also know as forced hot water.  As a result, AC powered hydronic pumps and valves are widely available at places like Home Depot and Lowe's.  Manufacturers like Wilo and Taco make great AC pumps that are dirt cheap at less than $100.   There are also a large number of AC powered controllers.  A great review of them is listed in the article called "Under Control, Solar Water Heating" by Chuck Marken & Doug Puffer. 

So why not do AC power?  Two reasons; efficiency and blackouts.  The AC powered systems are not particularly efficient.  The components selected are not designed for efficiency and as a result, they use a far amount of electric and are plugged in 24/7 (although not all components will be running the whole time).   For most people, this would not be a big deal, but I am not most people.  I spent almost 1 year investigating and reducing my electricity consumption (use 75% less than before) and I will be damned if I am going to reverse this trend.   The other consideration is blackouts.  The pump for the solar panel must run so that the propylene glycol never cooks in the sunlight, even in a blackout.  That is a problem for AC powered systems.

DC Powered:  With DC powered, there is no consumption of grid electricity and there is no fear of cooking the propylene glycol in a blackout.  It is also just really cool (IMHO) to power the solar hot water panels with solar electricity.  Unfortunately, there is a price to pay both in dollars and complexity.

DC powered systems are simply more expensive.  The solar panels might cost $200 and may need some additional electronics like a solar controller (more on that later).  Also the pump is more expensive, perhaps $300.  There seems to be only one manufacturer of this type of pump, Laing.  It is a very cool pump and can be run from 10 watts or less, versus 100 watts for a typical AC powered pump.  Also, there are very few DC powered controllers. 

All and all, it might end up costing me $1000 more to make my system DC powered versus AC powered, and I almost certainly will never see any return on this investment based on electricity savings.  So most people will probably want to have their system AC powered.  But for the one time in my life that I might be taking a hot shower during a blackout, I think that a DC powered system is the way to go.

For my system, I purchased two 10 watt panels from someone on ebay for a little more than $100.  It may end up costing me another $100 on brackets and things to mount them as well. The panels will be connected in series to produce 24 VDC.

The image shows the two solar PV panels mounted to a 2 foot wide piece of plywood that is part of some temporary work that I put together to test things.  It is really interesting to think that these two small panels will be enough to power the entire system.

Going forward, it is going to be DC powered.  One small bonus, I don't need to hire an electrician or have electrical inspections.  24 VDC is low voltage and not going to hurt anyone or start a fire.

The Plan

Solar hot water panels are normally placed on the roof.  This high location usually provides the most unobstructed access to sunlight, particularly here in New England with so many 40 foot tall trees.

But my plan is to put my solar hot water panels on the ground.  This has both advantages and disadvantages.

The primary advantage is that I don't have to mount panels way up on my roof and risk damaging the roof or having water leak from the piping through my roof and my walls.  Another advantage is that I don't have to find room on my roof for the solar hot water panels.  I already have an array of 16 solar PV panels up on the roof, and finding more space is a challenge (though not impossible).  Also, I had to remove a beautiful oak tree to create good sun conditions on that part of the roof, and I am not interested in taking down another oak tree.

The primary disadvantage (for my location) is lack of great solar coverage.  For the location that I have planned, there may be no solar coverage in the winter at all due to shading from trees.  Fortunately, the solar coverage is good for the other three seasons.  More about that in a separate post.  The other major disadvantage, is that the glass on the solar panels may get destroyed by snow falling off the roof above it.  You see, the location that I am choosing is right next to my house and snow falling off the roof may fall on my solar hot water panels.  This is particularly a concern because the location is right below my solar PV panels.  When snow comes off of the solar PV panels, it tends to come all at once in an avalanche.   It sounds like thunder when it happens.

  In any case, the plan is ground mounted solar hot water panels tilted at 45° to the ground.  I am committed to that at this point.

The History

Solar Hot Water systems are not some new piece of technology.  They have been around since at least the 1970s and many were installed during the oil crisis at that time.  Unfortunately, the price of oil went down and we all forgot about solar hot water.  The solar hot water companies went out of business and when people needed service on their panels, they could not get service.  But they could call their normal oil heat guy, and he said, take those stupid things down.

   People did, or worse yet, left the panels there, in disuse, and the technology naturally developed a reputation of "not working."  But actually, the panels did work, and do work. However, they are not maintenance-free like solar PV (electric) panels.  So an industry is needed for solar hot water.  That industry includes manufacturers, installers, and service personal.  Unfortunately, at this time, all three of those are in short supply.

  The best resource for the subject is "Solar Water Heating" by Bob Ramlow and Benjamin Nusz 2010.  They walk you through the history, economics, and the technology.

   Flat plate solar collectors with propylene glycol (antifreeze) are recommended by Ramlow and others for use in most of the USA.  There are other types of solar hot water collectors, but the advice was nearly universal in my literature study that flat plate collectors are the best choice for my application.  If there was ever any doubt in my mind, it was erased by the fact that all newly installed panels in my area of Massachusetts are flat plate type, and all the remaining old ones from the 1970s are also the flat plate type.

The Beginning

Insulated the house: Check
Reduced Electricity consumption by 75%: Check
Installed 3KW Solar PV for remaining 25%: Check
Bought Prius: Check
Riding Bicycle to work: Check

Hmmm... now what else should I do to reduce my carbon footprint, reduce pollution, and save money.  I know, how about installing solar panels that create hot water for things like showers and washing dishes.  It should save me money on home heating oil (read: Diesel Fuel) and be a good step in the right direction.

To be fair, I have been thinking about doing this for more than two years now.  About one year after the Solar PV panels were installed, I had scheduled an appointment with NexAmp to discuss having them install solar hot water panels.  Unfortunately, the day before they were to come to my house, the 2008 recession took hold, I lost my job, and naturally I canceled the appointment to focus on a job search, which fortunately only lasted two weeks.  But I was still too scared to part with any cash in family bank account.  Perhaps others can relate; perhaps not, if they are still unemployed, broke, and suffering.

But now I am back.  Feeling a little more confident.  With a little luck, the expenditure on the solar panels will pay for itself over the next 10 years or less.  Currently I am burning 1000 gallons of home heating oil per year (heat and hot water).  At $4/gallon, this is about $4000/year.  If I can reduce that by 25%, that is $1000/year.  With the expected cost of the system at $10,000, perhaps it will be a 10 year payback.  Not too bad.

But I am an engineer damn it, and a good one at that.  I am also very good with my hands.  I plan on building this thing myself and learn from the process.

Let it begin.