Tank Heat Storage
Solar collectors are used
to harvest the sun’s heat energy from rooftops, but solar collectors are just
the tip of the iceberg when it comes to using this renewable resource. Many
engineers ague that conserving heat with insulation is more cost effective than
alternative energy heating and DHW systems and I must agree that insulation
should be a primary concern, but insulation alone will not eliminate the need to
burn oil. Too much insulation can actually be counterproductive. Air stagnation
inside super insulated homes can pose a health risk.
We need well planed
dwellings and well planned retrofits with entire roofs devoted to collecting
heat and power from sunlight, AND we also need a place to store energy. Being
tied to the General Electric umbilical cord may seem like a comforting and
practical alternative to maintaining a charge on a rack of batteries with a
photovoltaic system, but storing the sun’s heat is another way of storing
The thermal mass of walls,
floor and furniture inside a house can be used to moderate temperature swings
for short time periods with the use of simple solar hot air circulating systems.
HOWEVER, long term heat storage for home heat and DHW systems require more
insulated thermal mass and a more versatile means of heat exchange. Let’s just
say there are advantages and disadvantages to pneumatic and hydronic heat
exchange systems, but I’d like to focus on hydronic systems that use multiple
heat storing tanks for this article.
Some very simplistic DHW
systems use only one tank for both a fossil fuel and solar DHW system. As you
can imagine heating a solar storage tank with oil or gas or electric is
counterproductive since heat exchange depends on a difference in temperature. In
other words if a fossil fuel DHW tank contains hot water than little or no hot
water will be collected from sunlight. This is why most solar hot water systems
have at least one tank dedicated to storing heat. In this way cold tap water may
be preheated before it enters the fossil fuel heating system.
Unfortunately the high cost of
conventional DHW systems use expensive storage tanks with internal heat exchange
coils. A 100 gallon solar hot water tank may be cost $1000. Now imagine what a
serious 500 gallon DHW / home heating system might cost. This is where low cost
DIY solar applications come to the rescue.
Large home made open storage
containers lined with EDPM, like this one built by Gary Reysa can be a very cost
effective. The ones I build are similar except I use an exterior 2x4 frame
packed with insulation and I use polyethylene plastic on the inside. As long as
the storage tanks remain below 150 F polyethylenes will hold up, however EDPM is
easier to work with. Next time I will use EDPM.
The illustrated 600 gallon
storage container is divided into four isolated 150 gallon containers that help
to stratify heat and increase collector efficiency. A solar heated glycol water
mix is circulated through a network of pipes imbedded in a cement slab at the
bottom of the storage container. This is where collected heat is exchanged into
the water storage containers. The network of copper pipes emerged in the top
layer of water in the tank are used to extract heat for DHW. This home made
storage container is used for closed loop heat exchange. A multi tank heat
exchange system based on the same CONCEPT of heat exchange could look like this:
twenty three water tanks
each have a 55 gallon capacity and hold over 5 tons of water.
They rest on a cement platform through which copper pipes have been
imbedded. Hot collector fluid (glycol +water) is circulated through the bottom
platform to transfer heat into the tanks. After the tanks are cemented to the
bottom platform a top layer of cement and heat exchange tubes are built onto the
top lids of the tanks. Water will never flow through these tanks since they are
only used to store and exchange heat.
As you know heat exchange is
driven by a difference in temperature. Since the tank bottoms are colder than
the tank tops collector heat should enter through the bottom platform and DHW
heat should exit through the top platform. The hottest tank will be the first tank
to receive collector heat and it will also be the last tank to receive heat for
DHW. A simple home heating system may also be implemented with this system by
blowing cold household air through the heat storage chamber. Where should air
enter the chamber and where should it leave?
A variation of closed loop
multi tank heat exchange involves placing heat exchange coils directly inside
the tanks. This method works best with small DHW systems but it requires a lot
of copper for a reasonable heat exchange rate. Let’s now take a close look at
a simple three tank closed loop heat exchange system.
Tank 1 is the warmest because it’s the first tank used to transfer collector heat. Tank 2 is the warmer tank. It will never get as hot as tank 1. Tank 3, the warm tank, is designed to suck the last bit of heat from the already cooled collector fluid. Collector fluid returned to the collector from this tank has a sufficient temperature difference to maintain efficient heat transfer.
This series of tanks is designed to preheat water in three successive stages. Tank 3 preheats water for tank 2 and tank 2 preheats water for tank 1. This minimizes heat loss in tank 3 and delivers the hottest possible preheated water. Notice that storage vault water is only used to transfer heat. Vault water never leaves the vault.
The above illustrations should only be used as conceptual guidelines in the
heat exchange process. Practical close loop systems require a large surface area
to exchange heat at a reasonable rate. When copper tubes are imbedded in a
cement platform the heat exchange rate is increased because the surface used for
heat exchange is increased. Copper is an excellent heat transfer medium but it
should be protected with polyurethane before imbedding in cement. PEX can also
be used to exchange heat and although the heat exchange rate is slower the cost
is less and a protective coating is not needed, but more PEX than copper will be
needed for an equivalent exchange.
are the alternatives to a closed loop heat exchange systems?
Thought you’d never
ask… Conventional, open loop, drain back systems have recently become more
popular than closed loop systems because they use less copper and less plumbing.
HOWEVER, many plumbers are still apprehensive about the freezing problems
associated with non glycol based systems. It’s true that improperly sloped
pipes trap pockets of water that never drain, and sometimes frozen water even
accumulates in areas that plumbers expect to drain.
can we use a drain back system without the worrying about freeze problems?
Well if we could be sure
all the water drains from the collector into a nice wide, well sloped pipe, our
worries would be over. Unfortunately 2” copper pipes cost about $10/ft.
about 2” PVC pipe?
Not bad for $1/ft but what about the heat, won’t it melt?
PVC starts getting soft
around 180 F so it’s not a good idea to use PVC inside a collector where
stagnation temperatures can reach 250 F. However PVC can be used to channel
solar heated water from collectors. If the temperature of solar heated water
ever exceeds 150 F the heat storage chamber is inadequate for the surface area
of the collectors.
OK we can use a 2” PVC pipe
to channel heated water from a drain back collector, but what about the copper
inside a collector. Is there some way we could do away with copper inside the
I will not discuss MTD collectors at this time, but will use the above
illustration to clarify the concept of open-loop, multi-tank, drain-back heat
storage. Notice that no heat transfer tubes are used. Notice also that the basic
concept of heat stratification is the same for open loop and closed loop systems
with the exception that water is heated directly here without the use of heat
transfer tubes. Multiple tanks trap heat better than single tanks by doing a
better job of separating hot water from cold water. No sense returning hot water
to a hot collector. As storage temperature approaches collector temperature the
rate of heat transfer begins to slow. Multiple tanks delay, storage saturation,
improve collector efficiency and generally extend the heat collection process.
Here is some graphical evidence that illustrated the heat stratification
process. Notice that the first tank to receive solar heated water climbed 30 F
higher than the last tank that returns water to the collector.
As you can see these tanks
rest on a heat exchange platform that’s used extract heat for DHW. A
circulator pump installed in the hot tank could be used to circulate hot water
through a radiant floor. The water, cooled through the radiant floor would than
be returned to the cold tank. In this way the hottest water would always be used
up first. Water to the hot tank is preheated by the water from the warm tank
which is preheated by water from the cold tank. This three tank system is only
being used to illustrate a concept. In reality a minimum of 10 tanks would be
used for home heating and DHW. Actual tanks hooked in series would look
something like this:
So now we have a system
with no heat exchange tubes in the collector and one set of heat exchange tubes
in the bottom slab for DHW. Since the tanks are sealed we can use a high
reservoir tank to increase the pressure on the pump thereby increasing the flow
rate. Now let’s see if we can build a practical home heating/DHW system with
no flow tubes in the collector, tank or floor. With sufficient tanks we should
be able to blow cold household air through an insulated storage tank chamber
filled with tanks of hot water. (Forced hot air)
What about DHW?
We need pressurized water fed from our water supply….. OR DO WE? If you think everything up to now has been odd hold onto your hat. I’m sure the building department and plumbers union will love this one. just an idea that could use a little more R&D but I want you to think about the possibilities of a solar space heating / domestic hot water system that does not require metallic heat transfer in the collector or in the storage system. This is just one tank that supplies DHW without a heat exchange tubes of any kind but it should be connected to a multi tank system for all the heat and hot water you would ever need. The tank would always be refilled as needed to give a constant supply of fresh hot water. Can you imagine additional applications for a system like this.
Do we really need oil to stay warm?
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