THERMOSIPHON JOINS SIMPLE DRAINBACK WITH BACKUP HOT WATER SOURCE
Solar energy isn’t evenly produced on a daily or seasonal basis. The
sun’s energy is only available during peak solar hours each day.
Further, the number of quality solar days each month is heavily
dependent on seasons. To be functional, a solar thermal system must
provide storage to even daily and seasonal fluctuations in available
energy. Further, a backup system is required for days where solar
energy simply isn’t available.
For most thermal systems, water is used as the solar storage medium.
The rule of thumb for storage is 1 – 1.5 gallons of storage for each
square foot of collector. For example, 2- 4’ by 8’ solar collectors
would represent 64 square feet of collector area. Appropriate storage
would be between 64 and 96 gallons of water. This amount of storage is
sufficient to match both the capacity of collector to produce energy and
the system’s ability to store and make available that energy.
As shown in the following Exhibit, hot water demand may be constant
(light blue bars), but solar supply is seasonally quite variable (dark
blue bars). This exhibit shows that for this residential application,
approximately 190,000 Btu per day is a reasonable estimate of energy
demand. Energy supply (dark bars) varies greatly with the seasons.
Summer production is double the production of spring and fall shoulder
months. During winter’s short and storm filled days, a solar thermal
system cannot be counted on for any significant contribution. Clearly,
a reliable backup source for hot water is required.
The goal of a solar thermal system is to provide reasonable storage and
adequate backup without compromising affordability. Joining a solar
system with a standard hot water system is a reasonable approach.
Mating a solar system with a traditional hot water system is
traditionally done in one of two ways. The most common connection has
the solar system acting as hot water “pre feed” for the regular hot
water system. The concept is that hot water flowing from the solar
system into the standard hot water heater will not require heating, thus
saving energy. A second standard approach is to mechanically move hot
water from the solar system to the domestic system using pumps and
controllers. This second strategy provides the dual benefits of
reducing standby energy losses and increasing solar storage. The
tradeoff for these advantages is the cost of a pump and control.
An alternative to joining solar and standard hot water heaters is a
thermosiphon design. The trick is to plumb the systems together so the
two naturally combine to increase solar storage without additional
complexity or expense. Joining solar hot water tank with an existing
hot water system using thermosiphon design extends solar storage without
the expense or complication of circulation pumps and controls. The
result is a low cost, elegantly simple and highly functional solar hot
water system with integrated hot water backup.
Thermosiphon design follows basic principles of fluid dynamics. The most
basic principle is that hot water rises and cold water falls.
Introducing heat to water causes water molecules to become excited and
expand. Heated water becomes less dense and comparatively lighter than
heavier cold water. Thermosiphoning harnesses this decreased density
and buoyancy of heated water to naturally induce fluid movement.
Shown above is an 8o gallon Simple Drainback solar hot water system (on
right) connected in a thermosiphon configuration to a 50 gallon propane
hot water heater. This configuration similarly applies to any electric
hot water heater.
The key to this configuration is allowing the coldest water from the
bottom of the standard hot water heater to drop down into the heat
exchange coil in the Simple Drainback. Cold water entering the heat
exchanger warms, rises and exits the top of the heat exchanger where it
dumps into the hot water tank’s standard inlet.
The Simple Drainback’s high temperature ‘load side’ design is ideal for
this type of energy maximizing configuration. The Simple Drainback’s
tall, slender and high temperature design promotes stratification. The
massive spiraled heat exchanger promotes thermosiphoning within the
stratified tank. The load side heat exchange configuration allows
potable water to naturally flow from the cleanout of the standard water
heater into the heat exchanger. Addition of a check valve in the supply
lines prohibits reverse thermosiphoning.
The above image on the right shows the concept for the thermosiphon
feed, while the image on the left shows the plumbed connection,
including check valve. Note that the domestic cold water feed is
connected via a “T” with the thermosiphoning feed from the bottom of the
existing hot water heater. Given demand for hot water, the house supply
pre-feeds potable water through the heat exchanger, as intended. The
additional cost of plumbing is negligible.
How well does a thermosiphoning system perform? The day the pictured
system was commissioned, water in the sink faucet was 145F at 3:30 P.M.
Since spring commissioning, the propane system has remained off.
Sometime in the fall, the propane backup will need to be restarted.
Until then, solar is supplying 100% of the hot water needs of this
family of 4. It is estimated that propane savings are on the order of
$50-60 per month.
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