For the last several years I've been experimenting with heating my pool
with solar energy. This year I build a prototype that heats my home's
domestic hot water via solar power as well. This is an explanation of the
graph my system generates daily showing the system's operation. I still
consider my system a prototype and it is still evolving. My next steps
will be to increase the size of my thermal mass and to use the stored
engergy to heat my house in addition to heating my pool and domestic
hot water. I'll keep my gas-fired convential tank water heater and my
gas-fired flash water heater as backup. Solar is the only option for heating
my pool.
This is an example graph, taken from late July:
This graph shows the temperatures at eight different points in my solar hot water system. I have a 150 gallon insulated water tank that serves as a solar mass. A Grundfos pump moves water from this tank, up to my two solar collectors, and back down to the tank when the solar panel is hotter than the tank water. The tank contains a heat exchanger immersed in the water.
The heat exchanger is 50' of 3/4" copper coiled tubing with the
coils spread out a little. The heat exchanger has domestic hot water
circulating through it. A second Grundfos pump does this, moving water
through the heat exchanger, into my old gas-fired hot water tank, and
through the hot water lines of my house.
This serves two purposes, (1) to move heat from the thermal mass to my
my house, and (2) to keep warm water in my hot water lines. This way
when someone turns on a hot water tap, they get water that is at least
warm right away.
All of my hot water lines are well insulated.
The control algorithm sets a fairly low target temperature between 11
PM and 7 AM, thus reducing the pump's energy consumption and the amount
that the energy in the thermal mass is depleted at night.
Both pumps are controlled my my ARM Linux system.
This is based on a Technologic
Solutions TS-7200. A Tri-M PC/104
relay and optoisolator card is plugged into, and sits on top of,
the TS-7200. A real-time-clock module is a second PC/104 card that is
in the PC/104 stack and sits on top of the Tri-M board.
I have four 4x10' solar panels that I purchased from a neighbor who was
throwing them out. They were 18 years old when I bought them, but of
good quality. The left two panels heat my pool, the right two panels
heat the thermal mass. The right two panels connect to the thermal mass
via 7/8" flexible hose (the kind used for dishwasher discharge water).
This is an internal arrangement. Eventually the thermal mass will be
larger, all the panels will heat it, the plumbing will be copper pipes,
and the pool, floors, and domestic hot water will all be heated by the
thermal mass.
The red line, "Panel Out", shows the internal air temperature of one
of the solar panels. You can see that it cools off a lot at night and
then warms quickly when the sun hits the panel in the morning. It gets
warmer during the day as the tank temperature, and thus the inlet water
temperature to the panel, increases.
The green line, "Hot Water", show the temperature of the pipe coming out of my old gas-fired water heater. It is this line that best shows the temperature of the water that my house is receiving.
The blue line, "Exch Out", show the temperature of the pipe coming out of my heat exchanger. This is most meaningful when the recirculation pump is running. When the pump is off, the temperature of this section of pipe drifts down towards the ambient temperature of the heater closet.
The purple line, "Exch In", show the temperature of the pipe going into the heat exchanger. This is most meaningful when the recirculation pump is running. When the pump is off, the temperature of this section of pipe drifts down towards the ambient temperature of the heater closet. When the pump is on, the difference between this line and the blue line ("Exch Out") show the amount that the domestic hot water is being warmed by the solar mass.
A 150 gallon
round stock tank surrounded with rigid polystyrene insulation and
covered with a tarp serves as my prototype thermal mass. It sites in
my yard, a few feet from the house. Having it separate from my house
allows me to make mistakes, spill water, etc., without causing problems
with my house. It was also a condition of the spousal building permit.
Here is a picture of the thermal mass with the top off. The copper tubing is the heat exchanger that heats our domestic hot water. The water pouring into the tank is coming from the solar panels. The outlet that takes the water out of the tank, to the pump, and then to the panels is about four inches above the base of the tank and not in this picture.
The aqua colored line, "Solar Tank", show the temperature of the water in the middle of the thermal mass. You can see that it drops down a little at night when the recirculation pump is on, but there is no heat being added from the sun, as heat as extracted. It rises on sunny days at energy moves from the roof-mounted solar panels to the tank. At night the tank temperature slowly decays as heat is lost through the insulated tank walls due to the difference in temperature of the tank and the outside air temperature.
The tank water heater is conventional and was installed about six
years ago. In the center of the picture you can see the red Grundfos
recirculation pump that was installed three years ago as a convenience
to keep the water in the hot water pipes warm. I plumbed in my heat
exchanger in such a way that I use this recirc pump to also move heat
from the thermal mass to the tank water heater and to the hot water lines.
On the right you see an electrical box that has two relays and two outlets
that are controlled by the relays. These relays are themselves controlled
by two of the eight relays on the Tri-M board in the ARM system.
The brown line on the graph at the top, "Flame", is often hard to see as
most summer days it is below the bottom of the graph. It's value comes
from a probe near the flame of the old gas-fired water heater. When it
pops up, it is because the household demand for hot water exceeded what
the solar system could deliver and the gas-fired water heater turned on
to heat the water. The graphed value is actually 40F less than what the
probe reads. This is so that it doesn't clutter the graph. The absolute
value of the air a foot from the water heater flame is not important.
Think of this line as the "gas on" line. In the graph above, you can
see the "Flame" curve just come up above the 40F mark for a few hours
in the afternoon. This is because it was a warm day and the ambient
temperature in the heater closet was high enough. If the flame had come
on, it would be sharper and higher curve.
The yellow plus signs at the bottom of the graph, "Solar P", shows when the pump is on that moves water from the tank, to the panels, and back. It is on during the day and shuts off in the late afternoon when the tank gets hot enough, and the sun gets low enough, that the system won't be heating the water.
The blue Xs at the bottom of the graph, "Recirc", show when the house's
recirculation pump is on. This moves water through the heat exchanger,
though the gas-fired water heater tank, through the main hot water pipe
in my house, and then back through a conventional "recirc return" pipe.
This comes on when the "Hot Water" value gets below a set value and
turns off if it reaches an upper limit (110F). The set value goes down
to 85F between 11PM and 7AM. During the day it is 100F, but but bumps up
a few degrees for the first five minutes of each hour. This hourly bump
keeps the hot water lines primed with warm water, for the convenience
of people in the house.
Notes and finer level of detail