Two PV Pumped Closed Loop Solar Water Heating Systems

This page covers two closed loop solar water heating systems that use a PV powered pump to circulate the working fluid. Both systems have been in operation for quite a while and work well.

The first is Doug's own system, and the 2nd is a system that Doug helped friend Bob install.

Doug's system

Bob's system

Doug's DIY Solar Water Heater

This Photovoltaic (PV) pumped hot water system has been working well, with no maintenance, for years on my house. I am now well past the point where the money I invested in the solar water heater equals the money I would have spent on electricity to heat water. Most of the year, we have more free hot water than we can use. Consider the fact that in the next five to eight years you are going to pay the cost of a solar water heater, whether you buy one or not. I kept costs down by doing all of the work myself, and buying a used collector panel, but still created a long lasting, efficient, high quality system.

I started by looking in the yellow pages under "Solar", and found a plumber in the nearest large city, with spare collector panels. I bought a 3`x13` aluminum collector with copper tubing and tempered glass cover for $100, guaranteed not to leak, but I tested it first, anyway. Assuming the collector needs to be freeze protected, a non-toxic antifreeze, usually propylene glycol, is pumped through the collector, to the heat exchanger tank.

Collector mounted on the roof. PV panel to drive circulator pump is visible at the upper left.

Solar water heater plumbing running through the attic.
The 12 volt DC circulator is mounted to the 2X4.
The pump used is no longer available, but pumps offered by Ivan Technologies (El Sid) or March Pumps would be good substitutes.

The simplest way to circulate the anti-freeze is with a 12 volt pump directly wired to a small photovoltaic panel, this eliminates the need for controls of any kind. When the sun is heating water, the pump runs. I bought a matched set of a 10 watt 12 volt solar electric panel, and bronze magnetic drive circulation pump from Zomeworks.
More on the PV pumped system here ...

The most expensive single component was the Heat Exchanger (HE) tank. I looked at what is available, and most are built around the ordinary glass lined steel water heaters, which have a limited lifetime. For less money, I had a stainless steel tank built with 50' of 3/4" soft copper tubing in the lower half as my Heat Exchanger. It holds 85 gallons, and acts as a pre-heater for the regular electric water heater. Non ferrous stainless will outlast me.

Proper sizing of the system is important. Plan on at least 20 gallons of storage tank size for each of the first four people and 15 gallons for each additional person per day. You should have at least 40 square feet of collector area for the first two family members, then add 12-14 square feet for each additional family member. Keep tank size at a ratio of 1.5 gallons or more to one square foot of collector area to prevent overheating.

Besides the pump, panel, HE tank, there are a few other parts to the system:
1) A pressure gauge (0-60psi) will let you know the closed loop has not lost it's charge of antifreeze.
2) A solar expansion tank allows the solar solution to expand as it heats.
3) A check valve above the tank to prevent thermosyphoning at night.
4) A pressure relief valve.
5) A hose valve at the lowest point for filling and draining. It really is a reliable, efficient system, we do no maintenance to it. I can read water temperature going in to my electric water heater, and most of the year, it is above the 120* setting of the thermostat.

Storage tank with internal heat exchanger.

It's your choice-you can invest in solar now, demonstrating your support for sustainable energy, and getting free hot water after your payback period, or continue to pay ever-increasing energy bills, which indicates your support for maintaining the status quo."

Diagram of solar water heating system

Click on diagram to enlarge

Doug

Nov 20, 2008

You can reach Doug at: dougkalmer AT gmail DOT com (replace AT with @, and DOT with a period)

Bob's DIY Solar Water Heater

My 17+ years of success with solar hot water made my friend Bob want a similar system for his new house. Through word of mouth, I heard about someone in Huntsville who had a four panel solar system free for the removal.

It was installed in the early 80`s and heated domestic hot water, had fan coil units for space heating, and heated an in ground pool. The systems installed then used electronic controls to operate valves and pumps, commonly the controls develop problems and parts and knowledgeable service techs are hard to find.

Bob and I spent a long hard day removing the panels, two 120 gallon tanks, three heat exchangers, much copper piping, valves, pumps, etc. We planned on using the components to assemble more modern systems using a Photovoltaic (PV) 12 volt panel directly wired to a DC pump to eliminate the controls and use of external electrical inputs- meaning stand alone systems using only solar energy to heat and circulate a non-toxic antifreeze to the Heat Exchanger (HE). The HE then uses only gravity to circulate tap water through it, heating the 120 gallons in the insulated tank. This is called a closed loop, PV pumped solar hot water system.

The two 40 sqft solar water heating collectors and
the PV panel that powers the pump.
The two collectors are connected in series.
I welded up the rack using 1" angle iron, my MIG running
from Bob`s generator. He has stand alone PV power,
and his system would not power the welder.
He poured the footers using tarpaper wrapped
into a cylinder for a form.

Click on pictures to enlarge

PV panel and pump in housing.
Bob trenched and buried the lines to the house, first covering
them in insulation, then enclosing them in PVC pipe.

There are two fluid loops:
In the collector loop, non-toxic antifreeze is pumped by the PV powered pump through the collectors, then underground to the house and through the inner tubes of the two heat exchangers, and then back to the collectors. The pump used in this loop is a Hartell MD-10-HEH 12 volt, brushless magnetic drive, PV direct.

An expansion tank is used to accommodate changes in the volume of the liquid in the loop with temperature changes. Valves are provided to allow Bob to just heat domestic water, or just flow the antifreeze through the PEX in the slab for space heating, or to do both at once. In the storage tank loop, the potable tank water in the heat exchanger is warmed by the collector water, and rises up the pipe that enters the top of the storage tank. Cool water from the bottom of the tank, being denser, flows by gravity
into the bottom of the heat exchanger to be warmed.

The circulation loop on the storage tank side uses only natural convection to circulate the fluid. In order for this to work well, the bottom of the tank must be elevated above the top of the heat exchangers.

Bob has tubing running through the concrete slab of his home, he can have the heated antifreeze either just heat domestic water, or the slab for space heating, or both. That's why he's using two panels, one panel and a smaller tank would be sufficient for just domestic hot water for families of up to five. Each panel is nearly 40 square feet. Bob never soldered copper pipe before, but he did a good job of reusing as much tubing as he could.

120 gallon storage tank.
Top of heat exchangers just visible.
Tank is elevated to allow thermosyphon circulation
of fluid in the tank circuit.

Click on pictures to enlarge

The two heat exchanger plumbed in parallel.

All pictures are before Bob insulated all piping and the HE. Bob's main expenses were for the 12v PV panel and pump. I used a 10 watt panel on my similar system, Bob chose a 30, I feel this is larger and more expensive than necessary. The pump is mounted close to the PV panel, Bob and I modified an ammo box to house it. I built the rack to support the panels. This sort of reliable system can be put together for considerably less cash than buying a complete system, it just requires hard work and the willingness to learn. Free heat and hot water from the sun!

Doug

Jan 13, 2008

You can reach Doug at: dougkalmer AT gmail DOT com (replace AT with @, and DOT with a period)

A small caution: The heat exchangers in Bob's system are single wall heat exchangers. This means that a leak in the heat exchanger wall can allow antifreeze to enter the potable water stream. This makes it VERY important to use non-toxic antifreeze in the system. Systems that use single wall heat exchangers should have labels that explain to future owners that non-toxic antifreeze must be used. Codes in some areas will prohibit single wall heat exchangers even with non-toxic antifreeze.
Efficient double wall heat exchangers are available, and are not that much more costly.

Gary

Doug's Other Projects

Doug has contributed a number of projects to Build-It-Solar covering a wide range of solar and renewable energy areas -- see them all...

Thanks very much to Doug for providing this material!