Closed Loop Antifreeze System Component
Ken Olson ©2001 Ken Olson
If you want a solar hot water system for your home and you live where it
freezes, this article is for you. If you’re installing your own system,
it will help you get the right parts for a system that works. If you’re
planning to hire a professional, it will help you know what you’re getting.
Solar Hot Water: A Primer covered the fundamentals
of solar hot water heating systems, including collectors, different types
of systems, and rules of thumb for sizing. In this article, I focus on one
system that is commonly used for solar hot water in freezing climates. You
will learn the principles and parts that make up the “closed loop antifreeze”
type solar water heating systems.
Fluid Basics
Fluid Flow
Rule of Thumb: For optimum flow rate through flat plate collectors, provide
about 0.015 gallon (0.057 l) per minute (gpm) for each square foot (0.09 m2)
of collector. That translates roughly into about 1/3 gallon (1.3 l) per minute
for each 3 by 8 foot (2.2 m2 collector, and 1/2 gallon (1.9 l) per minute
for each 4 by 8 foot (2.97 m2) collector.
This will result in a 15 to 20°F (8–11°C) rise in temperature
from inlet to outlet of the collector. Higher flow rates are unnecessary and
use more electricity, but are not detrimental to performance. Lower flow rates
result in less thermal efficiency because the collector runs hotter and loses
more heat to the ambient environment. If you are using evacuated tube collectors,
you could safely double the flow rate per square foot of collector.
Pressure & Head
A column of water 2.31 feet (0.7 m) high exerts a pressure of one pound per
square inch (psi).
Therefore, 2.31 feet of head is equal to 1 psi. If you ever need to do the
math to convert feet of head to psi, just divide feet of head by 2.31 or,
conversely, you can multiply psi by 2.31 to arrive at feet of head. head (in
feet) ÷ 2.31 = psi psi X 2.31 = head (in feet)
These equations are just two different ways of saying the very same thing—pressure. And the pressure is the force a pump must overcome. The static head is equal to zero in a pressurized, closed loop system completely filled with fluid. Gravity is not a factor because the pressure is equal throughout the system. The amount of force (pressure) of the fluid above the discharge or outlet side of the pump is counterbalanced by the amount of force (pressure) of the fluid above the suction or inlet side of the pump; net static head is zero.
Circulating pumps in pressurized closed loop systems are low head circulating pumps, since they only need to overcome dynamic head, which is usually quite low. As a result, they have very low power requirements.
Closed Loop Antifreeze Systems
“Closed loop” is a general term for any portion of a hydronic
heating system that is filled with a fluid at the time of installation. These
systems remain closed to renewed supplies of corrosive oxygen. Open loop systems
handle new water on a frequent basis, and must resist the deteriorating effects
of exposure to a recurring supply of oxygen.
A “closed loop antifreeze” system is nearly identical in design
to a conventional hydronic heating system; it simply uses solar collectors
in place of a boiler. A hydronic heating system is any system that uses a
fluid such as water, antifreeze, or oil as the medium of heat transfer. A
“boiler” is where the fluid is heated. A closed loop solar hot
water system includes a closed loop and an open loop. The solar collectors
are part of the closed loop. The open loop portion circulates the domestic
water to be heated.
The major parts of a closed loop, antifreeze type system include solar collectors,
circulating pumps, a differential control with sensors, heat exchangers, and
storage tank. Lesser but essential parts include an expansion tank, pressure
relief valve, check valve, drain/fill assembly, and pressure and temperature
gauges.
Flat plate solar thermal panels are connected in parallel, and usually installed
on a roof or mounted on a ground structure. The closed loop components can
be pre-assembled into a “module” that can be fastened to the wall
near the solar storage tank. Finally, the heat exchanger, solar storage tank,
and collectors are connected to the closed loop module. Let’s take a
closer look at the function of each of these components.
Collectors
Flat plate collectors are most commonly used for low-temperature applications
(up to 140°F; 60°C), such as residential domestic water heating and
pool heating. Water passes through parallel copper tubes bonded to a flat
copper sheet under glass, all enclosed in a weathertight insulated frame.
For higher temperature applications (over 140°F), evacuated tube collectors
are more efficient. The design of evacuated tube collectors reduces heat loss
caused by convection, radiation, and conduction. To locate a supplier of solar
collectors in your region, check your local yellow pages.
Circulating Pumps
Centrifugal-type circulating pumps are most commonly used in solar hot water
systems and hydronic heating applications. Centrifugal circulating pumps are
appropriate for their low power consumption, low maintenance, and high reliability.
They typically are made with cast iron, bronze, or stainless steel bodies.
For closed loop systems, (see Rust Never Sleeps,), lower cost, cast iron circulating
pumps are adequate. For the open loop part of the system, which circulates
a replenishing supply of water, a bronze circulating pump is necessary. Stainless
steel circulation pumps are used in pool systems and other applications where
chemicals are present.
Once you know whether your circulating pump is to operate in a closed loop,
open loop, or other particular environment, pump selection will be based on
head and flow requirements. Head is the pressure the circulating pump must
develop in order to create desired flow through the system. The overall pressure
a pump must create is determined by the height the water must be lifted and
the frictional resistance of the pipe.
Static head is pressure resulting from the vertical height and corresponding weight of the column of fluid in a system. The higher a pump must lift the fluid against gravity, the greater the static head it must develop. Dynamic head includes the frictional resistance of the fluid flowing through the pipe and fittings in the system. The pressure a pump must develop to overcome dynamic head varies with the size and length of the pipe, number of fittings and bends, and the flow rate and viscosity of the fluid.
Circulating pumps are typically categorized for low, medium, or high head applications. Low head applications have 3 to 10 feet (0.9–3 m) of head; medium head applications, 10 to 20 feet (3–6 m) of head; and high head applications, over 20 feet of head.
A pump curve is supplied for each pump by its manufacturer (see the pump curve diagram above). The flow rate, in gallons per minute is shown across the horizontal axis. The vertical axis shows the pressure as measured in “feet of head.” Each pump has its own curve, which shows the volume of flow it will create at any particular head it must overcome. If you refer to the pump curves above you will see that the Grundfos model 26-96 F will push 20 gallons (72 l) per minute (gpm) against a resistance of 10 feet (3.1 m) of head. Ten feet of head is equal to 4.3 psi (10 ÷ 2.31 feet per psi = 4.3 psi).
For the solar closed loop, you can select a low head circulating pump with
a cast iron or bronze body. You might consider the Grundfos model UPS 15-42
F to cover a wide margin of error. This model is a threespeed circulating
pump that allows you to select the most appropriate speed once installed.
A low head bronze pump will suffice for the water loop.
Whether open loop or closed loop; high, medium or low head; your local plumbing
supply house probably carries an appropriate pump without special order. Most
solar domestic hot water systems can be served by the Taco 008F, Grundfos
15-42 F, or Hartell MD-10IU pumps for the glycol closed loop; and by the Taco
006B, Grundfos 15-18 SU, or Hartell MD-3IU for the open loop domestic hot
water.
Controller & Sensors
The controller is the brains of the system. It tells the pump when to turn
on and off, based on collector and storage tank temperatures. All of its intelligence
is based on determining whether the collector outlet is sufficiently warmer
than the bottom of the tank to warrant turning the circulating pump on.
Sensors are located at the collector outlet, and at the bottom of the solar storage tank. These sensors are thermistors that change their resistance with temperature. The differential control compares the resistances of the two sensors. It turns the pump on when the collectors are sufficiently warmer (20°F; 11°C) than the bottom of the solar storage tank to collect useful heat.
The Independent Energy GL-30 is an example of a good differential control. It has an adjustable setting on the order of 5 to 25°F (3–14°C) temperature differential. These controls also have a high limit cut-out that will shut the system down once the tank reaches a predetermined high temperature, adjustable from 110 to 230°F (43–110°C). The GL-30 uses 10 K ohm sensors, which are the standard of the industry today. A 10 K sensor reads 10,000 ohms at 77°F (25°C). Temperature sensors must have the proper resistance to be compatible with a given controller. Sensors are available from the distributors who carry the controller you are using.
Heat Exchanger
The heat exchanger transfers heat from the solarheated closed loop to
the domestic water. Factors that increase heat transfer are:
• Greater surface area
• High thermal conductivity
• Maximum temperature differential between the two fluids
Heat exchangers may be categorized as single wall or double wall, which refers
to the number of barriers between the two fluids exchanging heat. Single wall
heat exchangers are usually not permitted in potable (drinkable) water systems
when a nonpotable heat transfer fluid is used. For example, systems that use
glycol should not use a single walled heat exchanger because of the potential
for contamination of the potable water.
Double wall heat exchangers are required to ensure that the heat transfer
fluid will not contaminate the potable water. The space between the two walls
of the heat exchanger is usually vented to permit detection of a leak.
A heat exchanger may be as simple as a copper coil within the storage tank,
where single wall heat exchangers are permitted. Solar heated water is circulated
through the coil with the help of a circulating pump. Heat is transferred
by natural convection to the water within the tank.
External heat exchangers are generally less costly than a custom solar tank
that includes an internal heat exchanger. External heat exchangers typically
require two circulating pumps—one for the collector loop and another
for the water loop. This uses more energy to run the second circulating pump,
but it is more effective than an internal heat exchanger that relies solely
on natural convection on the water side.
Heat transfer is driven by temperature differential. For this reason, heat
exchangers are installed in a counterflow configuration whereby the two
fluids flow in opposite directions through the heat exchanger. This maximizes
thermal heat exchange by maintaining the greatest temperature differential
between fluids.
Air Elimination
Air is your enemy in a closed loop system. Air pockets can stop or slow fluid
flow and defeat system performance. An air pocket on the suction side of the
pump can cause the pump to burn out. All free air must be removed from the
system when it is initially charged by the installer. A coin vent (you can
open it with a coin or small screwdriver) is installed at the highest point
in the system, which is usually at the outlet of the collectors. This allows
you to manually vent air from the top of the system, and aids the installer
or service technician in eliminating air from the system when charging it.
That is not the end of the saga on air. Over time, high temperatures and low
pressures tend to drive dissolved gases out of solution, forming air bubbles.
The air bubbles collect to form pockets in high spots within the plumbing,
particularly at 90 degree elbows and fittings where fluid turns downward.
Coin vents can be installed at any place within the system where air is likely
to collect. For large closed loop systems, air can also be eliminated automatically
by use of an air eliminator and vent. The air eliminator has a washboard-shaped
baffle to shake the air bubbles free from the fluid stream. A small reservoir
space above the baffle allows air bubbles to collect where they can be vented
out by either an automatic air vent or manually operated coin vent. Air eliminators
are usually unnecessary on small closed loop systems.
Selection of a heat exchanger is based on its capacity to transfer heat (in
BTUs per hour) produced by the solar collectors. The manufacturer or distributor
of the heat exchanger will specify which model is adequate, depending on the
total square footage of collector to be installed. They will also specify
a recommended minimum flow rate on the water side of the heat exchanger to
achieve an adequate rate of heat transfer.
Check Valve
A check valve permits fluid to flow in one direction only. It prevents heat
loss at night by convective flow from the warm storage tank to the cool collectors.
Check valves may be of the “swing” type or the “spring”
type.
Swing-type check valves should not be installed vertically upside-down, since
they can hang open. If you are powering your circulating pump directly from
a PV module, you should use the swing-type check valve. Low sun conditions
produce lower flow rates, which may not be strong enough to overcome a spring-type
check valve. For systems using AC circulating pumps, spring-type check valves
are preferred. The spring provides a positive action against thermosiphon
flow in either direction.
Expansion Tank
An expansion tank allows for the fluid in the closed loop to expand and contract
in the cycle of heating and cooling. Without the expansion tank, the plumbing
would easily burst when the fluid is heated.
Diaphragm-type expansion tanks use an internal bladder and pressurized air
chamber precharged at 12 to 15 psi. The solar-heated fluid expands in the
closed loop against the bladder and pressurized air chamber. As the fluid
contracts while cooling, the air chamber maintains pressure in the closed
loop.
Diaphragm-type expansion tanks may be installed in any orientation, but if
inverted such that the air chamber is above the fluid, it will continue to
function even when the bladder eventually fails.
The size of the expansion tank must be able to handle the expansion based
on the volume, coefficient of expansion, and range of temperature fluctuation.
These factors are considered in the rule-of-thumb recommendations below, which
you may use to estimate expansion tank size based on total fluid volume.
The size and number of collectors, and the size and length of piping and fittings
determine fluid volume. Use a #15 expansion tank for volumes up to 4.7 gallons
(18 l), and a #30 expansion tank for volumes up to 12.5 gallons (47 l). Multiple
expansion tanks can be used to increase capacity if necessary. Extrol diaphragm-type
expansion tanks are readily found in most plumbing supply houses.
Pressure Relief Valve
Every hydronic heating system must allow for protection against excessively
high pressures due to high temperatures. A pressure relief valve of 50 psi
is typically adequate to protect closed loop plumbing from excessive pressures.
Temperature/pressure relief valves are not commonly used in the closed loop
because high temperatures can be a frequent occurrence. Pressure-only relief
valves are most commonly used. Once one of these valves opens, it is wise
to replace it, since they often may not reseat, leaving a slow but persistent
leak. Pressure relief valves should be fitted with a vent tube to direct vented
fluid to a bucket or floor drain.
Gauges & Meters
A pressure gauge will tell you if the closed loop is within an acceptable
range of pressure. A typical system pressure is on the order of 12 to 15 psi.
So a gauge that registers up to 30 or 50 psi is suitable. The system pressure
normally does not need to exceed 25 psi. A pressure gauge is used as a diagnostic
tool to monitor the state of the glycol charge.
Two temperature gauges in the closed loop and one in the water loop are optional
indicators of system function. One gauge on each side of the heat exchanger
in the collector loop will show the temperature rise across the collectors
and the temperature change across the heat exchanger.
A temperature difference of 15 to 20°F (8–11°C) indicates effective
operation. One temperature gauge in the water loop between the exit of the
heat exchanger and the entry to the storage tank will display the current
temperature of solar heated water.
Select a temperature gauge with a range of 0 to 250 or 300°F (-18 to 120 or 150°C). A hot summer day may produce water temperatures exceeding 200°F (93°C), although normal high temperatures are usually around 180°F (82°C).
Antifreeze
The collector loop circulates an antifreeze solution. Propylene glycol is
the most common heat transfer fluid. It is a non-toxic substance, and more
commonly used as a food additive, though it is not considered a potable fluid.
Propylene glycol is usually mixed in a 50:50 solution with demineralized or
distilled water. Inhibitors may be added to increase the life of the fluid,
which breaks down over time due to overheating. It then forms a sludgy deposit
that can clog the collector loop, as well as reduce the solution’s effectiveness
as an antifreeze.
Ethylene glycol should never be used. It is the common antifreeze used in
automobile coolant systems. It is highly toxic, and will cause a great deal
of discomfort and death if consumed.
At the time of installation, the collector loop is charged to operating pressure
with a positive displacement pressure pump. Positive displacement pumps have
the capability of creating sufficient pressure to lift the fluid the full
height of the system, and bring the system to operating pressure, typically
about 20 psi. Positive displacement pumps also create sufficient suction head
to draw the charging fluid from the bucket.
The charging pump is not a fixture of the system. It is a separate piece of
service equipment used by the plumber or mechanical contractor. It is connected
to the drain/fill assembly in the collector loop, which consists of two boiler
drains with a shutoff valve between them. Alternatively, you can save yourself
a valve by replacing the shut-off valve of the drain/fill assembly with the
system’s check valve. This ensures that upon charging the system, fluid
flows in one direction only, expelling air from the system.
Solar Storage Tank
Two tanks are more efficient than one. A solar storage tank may be installed
in addition to your conventional hot water tank. Solar storage tanks are commercially
manufactured with four ports at the top of the tank. This makes plumbing easy
and convenient. You probably won’t find the solar storage tank in stock
locally unless you live near a metropolitan area, or there is a solar contractor
with a steady volume of work in your area. So plan to special order that tank.
You can, however, easily modify a standard tank to perform just as well. It’s
easy, and it doesn’t cost anything. But I’ll leave the details
for another time.
For those summer months when you can be satisfied with solar hot water alone,
you can install a "bypass valve assembly" between the solar storage
tank and the backup water heater. The solar bypass consists of three valves
(or two 3-way valves), which allow you to give your conventional water heater
a summer vacation and supply the house with solar heated water directly.
If you like that idea, you should add a measure of protection for those days
when you just might get water hotter than you can handle. A tempering valve
can keep you from getting scalded when your solar heated water is hotter than
you normally enjoy from a thermostatically controlled conventional tank. The
tempering valve is installed between you and your hot water system. With it,
you can set the desired maximum temperature of the water delivered to the
tap. Hot water enters one side, cold water, if necessary, enters from the
bottom and mixed water goes out to the tap.
Every component mentioned so far in this article, with the exception of the
solar collectors, solar storage tank, and heat exchangers, is commonly available
at your local plumbing or heating supply house. You can special order these
items, but you'll probably find more knowledgeable service at a solar supply
house. Of course, most solar supply houses will carry the common parts as
well, and can give you better advice when it comes to solar heating systems.
Putting the Parts Together
As you can see, closed loop antifreeze systems have quite a few components.
In this article, I’ve discussed the function of each component within
the system, along with principal selection criteria for each.
For the solar closed loop, you’ll need a low head cast iron or bronze
circulating pump, double wall heat exchanger, expansion tank, check valve,
one or two coin vents, pressure relief valve, pressure gauge, and two temperature
gauges.
For the open water loop, you’ll need a low head bronze circulating pump
and a temperature gauge. A differential control with two sensors will turn
the system on and off, and a solar storage tank will hold your daily catch.
Of course, you’ll need a bunch of fittings and valves, and don’t
forget an insulating blanket for that tank and pipe insulation to make it
efficient. With good plumbing skills and an understanding of how the parts
go together, you can find yourself in hot water too, from the sun that is.