Subject: The Thermal Cistern and The Solar Closet

From: "Lawrence F. London, Jr." <[email-masked]>
Date: Fri, 31 Dec 1999 02:46:20 GMT

From: Toby <[email-masked]>
Newsgroups: alt.solar.thermal,alt.energy.renewable
Subject: The thermal cistern
Date: Fri, 31 Dec 1999 00:52:10 GMT
Organization: Deja.com - Before you buy.

The Thermal Cistern

A thermal cistern is simuliar to a thermal closet. IF you^Òre not
familiar with a Thermal Closet, please read  Nick Pine^Òs paper at his
website at:    http://vu-vlsi.ee.vill.edu/~nick/solar/solar.html. Nick
says:

A "solar closet" is an insulated box filled with sealed containers of
water, with a solar air heater attached to one insulated side. In a
simplified solar space- and water-heating technique, a
low-thermal-mass
isolated sunspace heats a house on an average winter day, with an
average amount of sun. A higher-temperature, compact,
high-thermal-mass
sauna behind the sunspace provides domestic hot water and space heat
for the house during cloudy weather. 

Solar Closets  are advantageous over solar passive houses as Nick
explains:

Many solar houses are expensive and low-performing. Active systems can
perform well, but they tend to be expensive and complicated. "Trombe
walls," invented in 1881 (US Patent No. 246,626), by Edward Morse of
Salem, Massachusetts, are still being used in today's passive solar
houses. However, because they are so poorly insulated, Trombe walls
perform poorly on cloudy days... A person might well build a typical
passive solar house, and end up with a house that costs more than a
neighbor's, while only using 30% less oil for heating. Some passive
solar houses take 20 years to pay for their passive features, versus
non-solar houses. A direct-gain passive solar house typically has a
very large uninsulated window in a living area, with a masonry floor
in
front of that, to store solar heat. This works well in sunny climates,
with few cloudy days in a row. 

A direct-gain passive solar house works in southern climates.  Solar
Closet^Òs extend solar heating to middle climates, like Nick^Òs in
Pennsylvania; however, the solar closet doesn^Òt work in Northern
climates. such as Minnesota or Canada where there isn^Òt enough Sun in
a winter^Òs day or two or three to charge up a solar closet for 6
cloudless days.  Something different is needed to extend solar heating
into northern climates hence, the thermal cistern.

Thermal cisterns are like solar closets in that they store heat in
water and are insulated so that the heat stays in the cistern and can
be tapped into when needed. They are  dark like a solar closet so
that mold and mildew are hopefully elliminated. Thermal cisterns
differ
from solar closets in that they are larger and store heat for more
than
several cloudy days; rather, just like the ancient water cistern which
stored water for several months on end, the thermal cistern stores
heat
for the whole winter. Because the cistern is so large, the water can^Òt
be stored in sealed containers (as in a solar closet) something much
more substantial must be used. During the entire summer, the sun^Òs
heat, in any climate (even the northern), is stored  in the thermal
cistern;  so that in the winter, when it^Òs cold, we can pump it out
with our thermal cistern pump.

A thermal cistern pump can be built by running tubing (part of a
larger
loop of tubing) in the cistern. The rest of the tubing is looped into
a liquid to air heat exchanger (perhaps a car radiator) in the house.
Anti-Freeze (or water if anti-freeze bothers you)  is circulated
through this loop, and a fan blows air across the heat exchanger.

The type of  thermal cistern that stores the sun^Òs heat is called a
solar thermal cistern. A thermal cistern can also be used to store
coolth. Thomas Bligh proposed (and perhaps built) an underground
icebox to cool the University of Minnesota bookstore.  In the winter,
heat was sucked out of the water in this thermal cistern until it
turned into ice. In this case, the heat-exchanger was placed outside
in the cold weather, and the cold weather cools the anti-freeze in the
heat-exchanger and this gets circulated into the icebox. In the
summer, this ice was used to cool the bookstore with a thermal cistern
pump. Southern climates don^Òt get cold enough to turn this water into
ice, but they do get cold enough to cool the water down. This type of
thermal cistern would be a non-solar, thermal cistern since it doesn^Òt
use the sun^Òs heat at all.

*****Rough thermal calculations*********

If a thermal cistern is unaffordable, then all thermal calculations
will be worthless. But as a very rough estimate, a 40' by 8' by 5'
thermal cistern, heated to 88F, would be able to supply:

(93-68)F * 64 Btu/F/cuft * 40ft*8ft*5ft = 2500 kBtu's.

  (100kBtu for every degree above 68F..)

Suppose 2 people live in the house half the day each day, they
produce:

2*400 Btu/hr * 12 * 30 * 7 = 2000 kBtu

Suppose your utilities (lights, fridge) draw 10 amps @ 120V each
hour..each hour of the day i.e.
   10*120 W * 3.41 Btu/hr /1 Watt * 24 hr/day * 30day/month * 7 month
at 50% COP,
   gives 10,000 kBth

Suppose 7000 kBtu (a very rough estimate) could be supplied by solar
passive heating during these 7 months   i.e.. 1000 kBtu/month

1000 kBtu/month * 1 month/30 days = 33 kBtu/day and if only 50% of the
days are sunny, then we would need 66 kBtu/day. And Minnesota might
average (over these 7 months) about 400 Btu/sqft/day of solar
radiation
from the sun, so you would need a sunspace with 66 k/400 = 165 sqft. A
sunspace over the thermal chimney would be 8 * 40 = 320sqft (twice
that
much).

A well insulated house might have a thermal conductance of 200
Btu/hr/F. The outside temperature in Minnesota averages 40F each month
from Oct through April. IF you want to keep your house at 65F (a
little
cold, but then the people aren^Òt there all the time, so sometimes it
can be colder and others hotter), then your house loses heat to the
outside:

       (65-40)*200=5k Btu/hr  = 5k*24hr/day = 120k Btu/day =  3600k
Btu/month.

Over  7 months, you need about 25,000 k Btu/year of cooling.

(25,000  10000  2000 - 7000) = 6,000 kBtu would have to be supplied
by the thermal cistern. You would need:

           6,000 kBtu * 1 cistern/2500 Btu^Òs = 2 Thermal cisterns.

*********How to build a thermal cistern********

We will want to build a 14 wide by 6^Ò tall, strawbale wall (each bale
is: 14 by 2^Ò by ~4^Ò) in the middle of this trench. This will leave a
5 gap, on either side of the bale, between the bale and the trench
wall. Concrete will be poured into this gap; but, first, the bales
need to be secured from slipping to the side when the concrete is
poured.

Perhaps 2 by 6 boards could be temporarily placed in this gap, and
removed after the pour, while the concrete is still wet. Regardless of
whether or not 2 by 6^Òs are used,  I recommend using rebar . Bend
rebar into a 14 wide, staple  each side of the staple should be at
least 7^Ò2 tall. Pour 6 of concrete into the bottom of this trench, and
while it is still wet, insert the staple into the concrete 3 deep, so
that the rebar is totally embedded in the concrete and doesn^Òt
protrude into the ground where it will rust from ground moisture. The
staple ends should be sticking straight up. So that the rebar forms a
2-row grid, each rebar 2^Ò apart.

In the middle the trench, between the staple ends, stack 3 strawbales
on edge to make the strawbale wall in the middle of this trench
extending 8 above the ground surface level.  Before you place the
bales, you might want to place 6mil rolled sheeting (plastic) between
the rebar, so that the bale wall is surrounded by plastic. This
prevents the bales from wicking water from the concrete, and impairs
concrete from seeping into cracks between bales. I built the footer
for my strawbale greenhouse this way. .  Connect 1 rebar on 1 side of
the bale wall with another rebar on the opposite side using baling
wire, so that 2 rebars sandwich the bale wall.

 On both sides of this trench, temporarily lay bales flat. These are
forms for the stem wall which rises 14 above the ground surface.
Again, put plastic on these bale sides which will touch the concrete.
The plastic on top of the bale is secured by 6' spikes which will hold
them it in place, and the plastic then drapes over the bales edge and
slightly into the trench . Along the side of the bale which isn^Òt next
to the trench, pound 2^Ò long rebar, temporarily and partway into the
ground, every 2^Ò. Wire the top part of these rebars to the rebar
staples in the strawbale wall to help hold the wall in place.

Now, pour concrete into the gaps between the bale wall and the
trench/forms, continue pouring concrete on top of the bale wall
between the forms. After the concrete hardens, remove the strawbale
forms, and you will have a 2^Ò wide stem wall sticking 14 above the
ground surface. Be sure to place J-bolts into the concrete while it is
wet so that you can attach topplates for the roof.

The next week, have the backhoe dig out the dirt inside of the
rectangle. IT might be easier for a backhoe (with it^Òs blade) to dig
it out it if 1 side of the rectangle wasn^Òt concrete (build  a block
wall after it is dug out.). Pour a concrete floor. Seal with
thermoseal (and possibly mortar?).

Is this affordable?

Take a 1^Ò wide cross-section of our rectangle.
The concrete is about ½ ^Ñ thick.
Sides of Walls: 4*5^Ò
Bottom and Top of walls:  4*2^Ò
Floor: 8^Ò
Total: 1^Ò*36^Ò * ½^Ò  = 18 cuft
18cuft * 1 yd/27 cuft = 2/3 yd @ $60/yd = $40

There is 12 sqft of bale at $4/bale: $4.
16^Ò rebar plus J-Bolt plus baling wire:  --- $6

$50/foot of cistern

The 8^Ò wide End walls of the cistern would be
Sides of Walls: 4*5^Ò
Bottom and Top of walls:  4*2^Ò
Total: 8^Ò*28^Ò * ½^Ò  = 112 cuft
112cuft * 1 yd/27 cuft * $60/yd = $250 plus [8*($4+$6)=$80] = $330

So, if you had a 40^Ò long, 8^Ò wide, 5^Ò deep cistern, it would cost:
    40 ft*$50/ft + $330 = $2330  plus the roof.

The roof:

You could do a number of things. You could  build a 2 by 6 floor on
top of it and build a guest house

Or, you could build a solar sunspace and solar heater above it to
collect the heat from the sun. (see Nick Pines paper )

1. The cheapest way to roof it:
You could cinch aircraft cable across the cistern (attached to the j-
bolts). Lay horse fence on top of the aircraft cable. Then place bales
on top of it. Place furring strips between the bale^Òs polytwine. Screw
plywood to the top of it, and roof it.  Add an overhand and stucco the
ends of the bales

A 4^Ò by 12^Ò roof section would use:
3 sheets plywood: $30
6 bales: $24
Rolled Roofing: $30
Total: perhaps $100.

So the roof costs: $25/ft

40^Ò of roof:  40^Ò * $25/ft = $1000

Total: $2330 + $1000 = $3330

My heat pump cost $5000, so this is definitely affordable ....
2. The sunspace roof:

As before,  cinch aircraft cable across the cistern (attached to the
j-
bolts). Lay horse fence on top of the aircraft cable. Then place bales
on top of it.  Attach 2 by 4 topplates to the J-bolts and build walls
for a sunspace. You wouldn^Òt need to even put plywood on top of the
bales. You could just shovel dirt on top of the bales to seal the
bales
to prevent  air infiltration into cistern. This dirt on bale method
has
been used in huge potate housed, in Twin Falls, Idaho., for a century,
and I have heard of bale/dirt roofs lasting  40 years or so

3. The workshop roof:

As before,  cinch aircraft cable across the cistern (attached to the
j-
bolts). Lay horse fence on top of the aircraft cable. Then place bales
on top of it.  Attach 2 by 6 topplates to the J-bolts and build 2 by 6
10^Ò walls. Build a floor for the workshop above the bales by nailing 2
by 6 joists to the 2 by 6 walls.

Toby

Sent via Deja.com http://www.deja.com/
Before you buy.
Lawrence F. London, Jr.  Venaura Farm
http://metalab.unc.edu/intergarden
/permaculture /intergarden/orgfarm /ecolandtech
[email-masked]  [email-masked]