thermomax vs flat plates [RE-wrenches]

Tom Lane, Energy Conservation Services tom at ecs-solar.com
Wed Oct 16 06:07:10 PDT 2002


Chapter 5

SOLAR THERMAL COLLECTORS

Understanding Thermal Collectors and Averaging Ratings
FLAT PLATE BLACK CHROME COLLECTOR VERSUS EVACUATED TUBE COLLECTOR
The SunEarth IC-40 (40.81 sq ft) and Thermo Technologies Mazden 30
(Thermomax TMA 600) (49.31 sq ft) USA vacuum tube or evacuated tube
collectors SRCC rating sheets are on the opposite page.  DO NOT ASSUME that
the A,B,C,D, and E category ratings for clear day, mildly cloudy, and cloudy
days are a fixpoint for your site.  A solar water heating collector will
"move" through the A category to the C or D category during a single day of
operation.  If your ground water inlet temperature to storage is 45F to 55F
then you will start the day in the A category and move through  C and D
(during winter).  Compare SunEarth and Thermomax or Thermo Technologies
"performance" on clear days from early spring to late fall and from late
fall to early spring in the Northern Climates.
When you multiply the BTU's per day in a combination or a chosen category
times .2931 you get watts hrs per day.  Divide by 1000 and you get KWH per
day.  Let's assume .10 per KWH for electricity.  If the Thermomax collector
operated all day in cloudy weather in the D category it would 'collect'
12,000 BTU's per day.  Multiply 12,000 x .2931 results in 3517 BTU's divided
by 1000 equals 3.5 KWH per day x.10 per KWH equals $.35 per day.  The
SunEarth collector at 6000 BTU's per cloudy day results in $.175 per day.
Cold cloudy days result in little worth while energy collection no matter
what type of collector is used when you examine only the D rating for cloudy
days.  Rule of Thumb: If you cannot see shadows, there is no worthwhile
solar energy available.  In category C for water heating, on a clear day the
SunEarth collector collectors 43,000 BTU's per day ($1.27 per day) and
Thermomax collectors 40,000 BTU's per day ($1.17 per day).  I averaged
categories B,C, and D to get a more realistic rating from these categories
for the Fall to Spring average for a clear day and a mildly cloudy day.  In
Northern or Southern climates, late Spring to early Fall, averaging A, B,
and C is more realistic than looking at single 'categories' for solar hot
water heating.  I average A, B, C, and D for clear and mildly cloudy days
spring to fall.  I averaged B, C, D from late fall to early spring for
Northern Winter climates for a clear day and a mildly cloudy day.  Category
E is only for industrial process heating or air conditioning outside the
range of solar hot water or solar spacing heating.  The tables below are
averages of the clear days and mildly cloudy days from the SRCC categories.

      CLEAR DAY   MILDLY CLOUDY DAY
Collector Clear Day Category Spring to Fall Average: ABCD Fall to Spring
Average: BCD Spring to Fall Average: ABCD Fall to Spring Average: BCD
Mazden 30 40,000 BTU/Day 38,670 BTU/Day 29,250 BTU/Day 28,000 BTU/Day
SunEarth IC-40 47,500 BTU/Day 40,000 BTU/Day 30,500 BTU/Day 27,000 BTU/Day


Florida Solar Energy Center Comparison SunEarth IC 40 vs Thermomax TMA 600)
The Florida Solar Energy Center's hot water intermediate temperature rating
is with 122°F test water going into the collector.  This immediate rating is
the FSEC rating category for solar water heating on clear days.  The
SunEarth delivers 39,800 BTU's per day or 975 BTU/sq ft.  The Thermomax
delivers 37,300 BTU per day or 757 BTU/ sq ft per day.   The pool collector
or low temperature rating is with a 95° inlet temperature. Thermomax
collects 40,200 BTU's per day versus SunEarth's 44,900 BTU's per day at this
rating.  It is only at the 212°F inlet test temperature at the high
temperature rating that Thermomax outshines SunEarth.  Thermomax collectors
25,500 BTU's per day versus SunEarth's 16,400 BTU's per day.  Understanding
where your collector and system will really "operate" is important when
choosing the collector.
Evacuated Tube Collectors
Thermomax Technologies is one evacuated tube technology that has a long
history.  These collectors consist of a heat pipe inside a sealed vacuum
tube that uses alcohol as the heat transfer media.  Water or glycol flows
through the manifold and picks up heat from the heat pipes.  The heat of the
water or glycol is pumped to a conventional solar storage tank or a heat
exchange tank.  The heat pipe acts by a capillary wick structure and a small
amount of vaporizable fluid.  The evaporating condensing cycle accepts solar
thermal energy to evaporate the liquid and releases the latent heat by
condensation at a heat sink region.  This is repeated continuously by a
return feed of the condensed fluid back to the heat zone.  In the 'solar
collector' the condensation zone is at a higher level than the evaporation.
The transport medium or condensation zone returns to the evaporation zone
under the influence of gravity.  The operating temperature has to be the
maximum temperature of the heat transfer fluid.  I recommend that you buy
completely engineered systems from heat pipe manufacturers.
Building an evacuated tubular solar collector and maintenance of its vacuum
has been solved by Thermomax's use of heat pipes instead of direct fluid
transfer in and out of the evacuated by the storage heat exchange medium.
No one has been able to manufacturer evacuated tubes that last long term
without using heat pipes.  The heat pipe technology has solved numerous
problems associated with this technology.  Increase the amperage on all PV
modules with March 809 12 volt DC pumps listed in this manual by .3 to .5
amps with evacuated tube collectors.

Flat Plate Collectors Versus Evacuated Tubes
"Don't use a bazooka to hunt rabbits" - especially expensive bazookas with
high maintenance (i.e. evacuated tubes) when only flat plat collectors are
needed. Selective coated black crystal, black chrome, or black painted semi
selective coatings (socoat) on absorbers of flat plate collectors are the
best choice for heating water to 140°F to 160°F.  They typically come in 3'
x 8', 4 x 6½',  4' x 8', 4' x 10', and 4' x 13'  sizes.  Evacuated tube
collectors are typically needed only when water temperatures from 180°F to
211°F are needed.  Concentrating collectors are to be used when 212°F or
higher temperatures are required.  Evacuated tube collectors seem "sexier"
and hi-tech and the literature promises dramatically increased performance
that research and real world experience does not support.  The flat plate
selective coated collectors' efficiency performance curve (to heat water) is
typically much better in all regions of the U.S.A.  Unless you want to heat
water over 180°F in the summer and over 160°F in the winter there is no
significant performance increase in using evacuated tube collectors.  Even
at extreme mountain altitudes in cold windy conditions, selective coated
flat plates will equal or outperform evacuated tube collectors in heating
water from 120° to 140°F.  The big deception is that evacuated tubes will
heat water much better under extremely cloudy conditions.  They will collect
some heat during these conditions in the tubes, however, few BTU's will be
transferred to storage.  If you cannot see a shadow being cast (less than
200 watts per meter square or 80 BTU's per square foot); there is little
thermal energy to be gained.  At 1000 watts per square meter, there are 350
BTU's per sq ft in full sun conditions.  Hardly any thermal energy is worth
collecting at less than 80 BTU's per sq ft where the evacuated tubes claim
to outperform flat plate collectors.  Evacuated tubes have trouble shedding
ice and snow in the winter - ice crystals freeze solid to the reflectors.
Flat plate collectors easily shed snow and ice if the tilt is over 30°.
Evacuated tubes are expensive and more complicated in their construction and
are prone to leaking seals and storage tanks overheating.  Evacuated tubes
have many long term problems that make them a poor choice for heating water
below 160°F.  Evacuate tubes have an unpleasing look on the roof to most
homeowners.  Alternatively, flat plate collectors can be made to look like
skylights.  Evacuated tubes require more expensive high temperature
insulation for the pipes than flat plate collectors.


----- Original Message -----
From: "Jay Peltz, Peltz Power" <jay at asis.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, October 08, 2002 11:05 PM
Subject: Re: thermomax info [RE-wrenches]


> Hi All,
>
> Thanks for the info.
>
> 1 more question, where do I buy them?
>
> Thanks again,
>
> jay
>
> peltz power
>
> Smitty wrote:
>
> > As I recall, Todd is right about the performance. I haven't played with
the
> > tubes for quite a while. Back when the earth was green, we saw quite a
few loose
> > their vacuum. Consequently, in the summer in New Mexico, the glass got
quite hot
> > and if rained on in the sunshine, which can happen here in the monsoon
season,
> > the tubes were prone to exploding, or shattering violently. Witnessed
one
> > myself. I kinda went hmmm. Has anybody else seen this and or has it been
> > corrected? It may be related to our NM climate.  Smitty  AAA Solar
Supply
> >
> > "Todd Cory, Mt. Shasta Energy Services" wrote:
> >
> > > I recall HP has a similar set up and already reported the results. If
my
> > > brain is functioning correctly the article reported that in sunny
conditions
> > > the BUT output was very similar (maybe the flat plate did slightly
better
> > > even), but in cloudy /low light conditions the evacuated tube system
did
> > > better.
> > >
> > > Todd
> > >
> > > Lawrence Elliott wrote:
> > >
> > > > I've been told that Portland Community College and Home Power
Central
> > > > have identical installations of a flat plate collector and a 20 tube
> > > > Thermomax mounted side by side.
> > > > I am looking forward to some precise data on how each performs in
two
> > > > different geographic locations.
> > > > Would be a great Home Power article.
> > >
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