Photo-voltaic (PV) versus Solar Hot Water Systems.
PV systems use silicon cells to create electricity
from sunlight, while solar hot water panels use dark
or selective surfaces to collect heat from the sun's
radiation.  Based on return on investment, solar
hot water systems usually provide a stronger
financial return than photo-voltaic (PV) systems.  
Part of that advantage is realized by the
efficiency of the panels.  Solar hot water
collectors are roughly 66% efficient in harvesting
the sun's energy, while most PV panels are still
below 20% efficient.  Many are below 15%
efficient.  If you choose higher efficiency PV
panels, they cost more.  The bottom line is that
you need three to four times the surface area, to
collect the same equivalent energy as a solar
thermal system.  PV systems cost considerably
more than solar hot water systems, so you would
need significantly more capital, to make such an
improvement to your home or business.  Certainly,
PV systems do capture real energy, and I am of a
you get what you get mentality, so yes, they do
work and work well, they are, however,
considerably more costly to install than a solar
thermal system.

Flat Plate Panels versus Evacuated Tubes
If the prior discussion doesn't get the juices
flowing, this discussion may get some folks to
come out of the woodwork.  That is great.  Let's
speak about your findings out there.  Please feel
free to contact me via phone or email or our
"Contact Us" page.

Flat Plate Panels come in two basic types, either
parallel tubes with headers, or serpentine.  
Basically they use a painted or specially coated
metal surface, with a tubing configuration to carry
away the collected heat.  All this is housed in an
aluminum frame, insulated behind and on the
sides, and it has a glass cover.  

Evacuated Tubes have several designs.  We will
not go in too deep here, because of the many
proprietary approaches.  Basically, the glass tube
is shaped like a test tube.  Now imagine it double
walled, and still open on one end.   The gap
between the double wall construction is
evacuated of all air molecules.  This places a very
good insulation layer between the collector fins (in
the center of the tube) and the outside air
temperatures.  In the center of the tube could be
a heat pipe or a U-tube arrangement, dependant
on make and model.  

With the U-tube design, the solar heat transfer fluid
actually flows down through the U-tube, which is
inside the insulated evacuated tube.  With the
heat pipe design the fluid remains up in the
manifold, and the heat pipe transfers solar energy
up into the manifold and the passing heat transfer
fluid.  From a design perspective, I give the heat
pipe version the nod.  It has some mechanical
advantages, in my opinion.  However, the U-tube
models are typically a bit more efficient at
collecting energy.  My issue with the U-tubes is
that the tubes are small, and, over time, they can
collect sediment in the bottom, and could
eventually clog, so, from a maintenance
perspective, I like the heat pipes better.

Extremely Cold Climates  OK, so this is where the
evacuated tubes have an opportunity to beat
flat plates hands down.  However, extremely cold
climates might be more like Canada or Alaska,
than upstate NY.  The evacuated tube's insulation
is better than an insulated box with a large glass
cover, so the flat plates won't achieve the
temperatures that evacuated tubes will on a very
cold day.  This performance is also the same in
windy conditions.  Flat plates have vent holes to
reduce internal condensation.  Cold wind will
cause drafts inside the flat plates, steal energy
and decrease overall efficiency, while the
evacuated tubes would be significantly less prone
to these losses.

Snow Shedding  Sorry, the evacuated tubes have
some serious issues here.  Tip your flat plates up to
about 45-50 degrees and they will shed snow well.  
Less angle, and you will need some melting to
assist the shedding.  With evacuated tubes, the
snow can get rapped around the tubes and also
attached to the mounting frame and the roof, so
the snow won't slide very readily.  The improved
insulation quality of the evacuated tube also
reduces the chance of energy radiating out, to
melt snow, so you will likely have more downtime
with snow covered evacuated tubes, than with
flat plates.  

Mounting  I give flat plates the nod here.  Several
of the evacuated tubes I have seen come with
some pretty crappy frames, somewhat light duty,
in my opinion.  Please share any brands of
evacuated tubes that come with robust
mounting frames, as I have yet to come across
any.  

Aesthetics  On this one, I prefer the flat plates
again.  They basically look like skylights when flush
mounted.  When elevated, you could argue that
they are both equally unattractive.

Cost  Flat plates typically cost less than most
evacuated tubes.  Beware of low cost evacuated
tubes, as they will likely be less efficient at
collecting energy than the flat plates.  Before you
buy your solar collectors be sure to visit the SRCC
Rating page to verify their performance.  You can
compare them to other offerings (and you can
compare flat plates to evacuated tubes there as
well).  The higher quality evacuated tubes can
cost considerably more than flat plates.  This cost
may be justified by the desired end use of the
energy, but again, the cost premium can be very
steep.

Maintenance  Again, flat plates are very simple,
and not at all demanding.  They can last 30 years
plus with 5-7 year antifreeze change intervals.  
Evacuated tubes are known to go bad.  Most
have an internal coating that changes color
when the vacuum seal is lost.  This necessitates
replacement of the tube.  In some cases,
especially with U-tubes, that may require the
entire system to be drained.  Long-term
availability of the tubes should also be a concern;
what if your model is discontinued?

Overall energy output  Higher quality evacuated
tubes have the capability to provide higher water
temperatures than flat plate collectors.  I do find
this to be true.  However, it is my opinion, and
backed by some research data that I am privy to,
that they may achieve this at an expense of
water volume (a gallon-per-minute flow rate that
is lower than the flat plates).  So the actual BTUs
produced by each may be quite comparable, if
flow rate is also included in the equation.  Also, be
careful if you are going the evacuated tube
route.  Some manufacturers manifolds are
extremely constricted and may not support series
connections of more than one or two sets  of
tubes.  Do your homework before selecting a
product.  

Cloudy Conditions  Evacuated tubes perform
better than flat plate collectors during cloudy
conditions and during early morning and late
evening.  I find this to be true, but for the majority
of a long sunny stretch, they may be strongly
outperformed by the flat plates, especially with
warmer outdoor temperatures.

Hybrid Arrays  Certainly, a robust and
well-designed combination of both flat plates and
evacuated tubes is an attractive opportunity, in
my opinion.  Flat plates could lift the temperatures
up to their limit, while the finishing stage uses some
evacuated tubes, to further elevate
temperatures.  I have been involved with some
testing of such arrangements, but with some
lower-quality evacuated tubes.  I am not closed
to this being the ultimate space heating setup.  It
would also provide some economies, compared
to purchasing all evacuated tubes.  

Solar Space Heating  Solar space heating is a
viable approach to lowering your heating fuel
costs.  However, It requires a good, sound design,
and you will still require another form of space
heating, in cases of long stretches of cloudy
weather and storms.  Does it work?  Yes, when the
sun shines.  This past November,in the Hudson
Valley, it seemed like we had no sun for the entire
month.  In contrast, February is a particularly good
month for solar, even though it is cold, due to
rather clear skies.  My 1,100 square foot pole barn
has solar assisted radiant slab with an oil boiler.  It
has 128 square feet of flat plate collectors
mounted vertically, on a South-facing wall.  I
consider this setup undersized for its application.  
However, I have seen 95-100 degrees F coming in
from the panels in February, and it gets circulated
right through the slab.  If the sun stays out, my
boiler is disabled, and solar heats the concrete all
day.  This arrangement has reduced my oil costs
by somewhere around 50%.

I typically recommend a minimum of four to six
panels for space heating applications.  Eight to
twelve would be awesome.  You could likely go
beyond 20 panels, and still need some additional
heat sources, based on the sun's availability.  It is
not recommended to try to go for eliminating
100% of your heating with solar, because in
non-heating months, you will have all those
panels, and nowhere to use all of the heat that
the system could provide (unless you have a
pool).  Remember that you would also need a
heat delivery method that can make use of solar
system's typical winter output temperatures
(maybe 90 to 110 degrees F, which is far less than
180F sent through most hot water baseboard
systems).   Radiant heating is the answer here, with
slabs and Gyp-crete pours on upper floors being
best.  Above sub-floor radiant and radiant
radiators are also possibilities.  Radiant radiators are
quite costly.  Their BTU output must also be
de-rated if you plan on heating with lower
temperature water.  Radiant zones with tubing
installed below sub floors may not match up with
solar as well, due to the higher design
temperatures required for these systems (typically
about 140 degrees F.

Very Important:  Do not expect too much from
your system!
Rome was not built in a day.  In any case,
installing a solar energy system is the right thing to
do and is a step in the right direction.  With an
entry level system, at least you can get 75% of
your domestic hot water heated for free.  
Remember, the best financial investment will not
be a system that handles 100% of your heating
energy needs, due to equipment costs, space
constraints, etc.   

APEX Thermal Services can assist you in an honest
fashion, to determine if your goals are obtainable.  
We can definitely design you  a basic or complex
system to assist in reducing your utility bills.  If you
have radiant heated floors or slabs, we can also
assist with some solar space heating to further
reduce your fuel costs.  If you would like your pool
heated with energy from the sun, you should
definitely give us a call!  You can also reach us
through our "Contact Us" page.

Again, we are here to help you achieve your
conservation goals.  Please contact us before you
design your energy efficient home.

Alan J. Paul, CEM, Founder
APEX Thermal Services
tel: 845 430 5825
email: AJPaul@apexthermal.com


Drain-back versus Pressurized Glycol Systems
Again, another comparison.  Which one is better?
It Depends!

In a nutshell, the solar loop (includes panels tanks
and connecting piping) within drain-back systems
is only partially filled with heat transfer fluid.  This
design allows the fluid to reside in a dedicated
tank inside, if the solar pump is not running.  This
design ultimately protects the heat transfer fluid
and the solar tank from over-temperature
conditions.  More on this later!

The solar loop within pressurized glycol systems
(which includes the collectors) is completely filled
and pressurized with heat transfer fluid.  This
provides less protection from overheating the
heat transfer fluid, which can accelerate the
breakdown of the antifreeze.  This type of  system
can benefit from a larger than average solar tank,
which would help avoid over-temperature
conditions.  In addition, a heat dump of some
type, can be designed into the system, allowing
excess or unneeded heat to be rejected to the
outdoors, into a radiant slab, into a pool, etc.  
Overheating typically can occur when occupants
go away on vacation.  If no hot water is used in
the house, solar tank temperatures will continue to
rise and rise, so a heat dump is one possible
solution.  Some solar controllers have a vacation
setting, which operates the pumps at night, to
cool the solar tank(s) to reduce/eliminate the
potential for overheating.

Basically, the design of your home and the
location of the panels and tanks, as well as the
piping path (drain-back systems need all piping
pitched downhill, toward the drain back tank) may
determine which type system you install.  Also, if
you desire a PV-pumped system, you will most
likely need to go with pressurized glycol system  
(PV pumps are not capable of lifting the weight of
the fluid as required within drain back systems.  

Piping
Drain Back systems are typically more piping
intensive than pressurized glycol systems.  But, if
you are planning on installing a heat dump on
your pressurized systems, the amount of piping
can be closer to equal or could exceed that of
drain back systems.

Myself and other professionals give drain-back
systems the nod for performance.  Disadvantages
are that the panels and piping must be tilted
slightly(to assist in fluid draining.  It is could
potentially be noticeable from the ground, if you
stare long enough.

Advantages to installing pressurized glycol systems
include system simplicity, smaller pumps, pumping
power, PV driven pumping becomes an option,
and a potential to use smaller piping to panels.  


Contact APEX Thermal Services to help you
decide which system is best for your needs


Alan J. Paul, CEM, Founder
APEX Thermal Services
tel: 845 430 5825
email: AJPaul@apexthermal.com

Link to our "Contact Us" page

Link to our "Solar Pool Heating" page

Link to our "Home" page

             Alan J. Paul, the Founder of
        APEX Thermal Services compares:
1) Solar Electric vs. Solar Hot Water Systems
2) Flat Plate vs. Evacuated Tube Collectors
3) Drain-back vs. Pressurized Glycol Systems