A $1000 solar water heating

http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#1KSolarWater
Full Details ...

Gary


As of late May 2009, the system has been in operation for about 9 months and has performed well and reliably, achieving solar fractions over 90% over our cold Montana winter.
This is a cut at $1000, all climate, high performance, long life, low maintenance, and easy to build solar water heating system.

It combines one of the two collector designs just below with a very simple drain back scheme. The system is unique in that it combines drain back and storage tanks into a single, large, inexpensive, non-pressurized tank. A unique heat exchanger that is inexpensive and very efficient is used.

Extensive build, testing, and performance information is provided ...

http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
$1000 Solar Water Heater --Overview



An attempt at a $1000,

cold climate,

high performance,

long life,

low maintenance,

easy to build

solar water heater.



This system can save $300+ per year in energy costs and reduce CO2 emissions by 2 tons. Full and free construction plans provided below -- it will cost about $1000 in materials.


As of late April 2009 we have lived with this system for 8 months. It has been trouble free and provided a solar fraction of 94% over the cold Montana winter. Aside from the failure of an off the shelf controller, it has required no maintenance. The drain back system has seen temperatures down to -30F with not even a hint of a problem.

I believe that the system has been and will continue to be the functional and thermal equivalent of commercial systems that seem to be going for around $7000+ these days.

Directory -- for the system overview provided on this page:

This page gives an overview of the $1K Solar Water Heating System.

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Objectives for the system

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Overview of the System Design

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Collectors -- Two Alternatives

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Storage tank and heat exchanger

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Plumbing

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Pump and Controls

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Federal rebates


Important: This page is just a quick overview of the system, but there are 20+ pages covering the design, construction, testing, cost, and performance of the system in great detail -- see this ROAD MAP for all of the gory details.




Objectives for the System

The objectives for this project are to design and build a domestic solar water heating system that:

1.

Costs less than $1000 using all new high quality parts and materials.
2.

Has a long life with little maintenance
3.

Works well in a cold, 4 season climate.
4.

Performs well, providing a high fraction of solar heated water for the full year.
5.

Is easy to build using commonly available materials.
6.

Does not look ugly

This is a fairly formidable set of goals given that commercial systems for cold climates often cost 5 to 8 times the $1000 target.



To accomplish the goals, the design uses these somewhat unique features:

1.

Two collector designs are covered -- either one can be used -- both are easy to build:

The first collector design uses low cost PEX tubing instead of copper to pick up heat from aluminum fins. The resulting collector costs less than 1/5th of a high quality commercial collector using copper pipe and copper fins, while delivering about 84% of the performance of an all copper collector.
The 2nd collector design uses a hybrid copper tube and aluminum fin combination. This collector option adds only a modest amount to the cost, and performs within about 4% of commercial collectors.

2.

The storage tank is non-pressurized, plywood box framed with 2 by lumber, and lined with EPDM rubber sheeting. This design allows a large storage tank to handle several cloudy days, and is inexpensive and easy to build. Tanks of this type have been in use since before the 80's, and have a proven track record.

3.

The heat exchanger is a large coil of plastic pipe that is immersed in the storage tank. The coil itself stores enough water (9 gallons) to satisfy most hot water demands, so performance loss due to the heat exchanger is usually zero. This concept comes out of another unique solar water heater that Nick Pine and I have been working on that may also meet the objectives listed above.

4.

The system is very simple, consisting of only: 1) the collector, 2) the storage tank with immersed plastic pipe coil heat exchanger, and 3) a single pump and controller. The low part count and the simplicity of the part designs reduces cost and makes the system easier to build, understand, and maintain.

5.

Both the collector and the storage tank can be oversized for very little extra cost -- this provides improved winter performance for a better year round solar fraction.

Commercially installed solar water heating systems typically cost $8000 -- this system can provide the same performance at $1000.


Overview of the Design





The pump (P) pumps water from the bottom of the large storage tank, up through the collector to be heated, and then back to the top of the storage tank. Incoming cold water from the street passes through the large pipe coil immersed in the storage tank and is heated by the hot storage tank water before it gets to your existing hot water tank (which provides backup heating when needed). The three valves provide for bypassing and isolating the solar tank for maintenance. There is a controller (not shown in the diagram) that turns the pump on only when the collector is hotter than the storage tank water -- this is an off the shelf item.



Our system uses both more collector area and more storage area than "normal" commercial systems use. This should improve winter performance and result in a higher year round solar fraction. Since you are building the collector and tank, the added materials cost and effort to oversize these elements is minimal. The collectors are also tilted at a steep angle to improve winter performance, and reduce summer overheating.



The heat storage is a large tank lined with EPDM rubber sheet (pond liner). The tank provides more storage capacity than usual for more cloudy day reserve and more thermal inertia.



The system uses a unique heat exchanger consisting of a LARGE pipe coil that is immersed in the heat storage tank. The pipe coil preheats water headed for your conventional hot water heater. There is enough hot water stored right in the immersed pipe coil to support a 15 minute shower -- after the hot water in the pipe coil is exhausted, the pipe coil acts as a conventional heat exchanger, picking up heat from the heat storage tank.



While the experience so far has been good, I still consider this to be an experimental design. If you build it, 1) you are taking some risk that some design defect may show up later, and 2) send me pictures and a description of what you build!!


Collector Size and Orientation

Typical solar water heating systems tend to do quite well in the summer and not as well in the winter. The reasons for this are: 1) less sun and more clouds in the winter, 2) temperatures are colder, so losses are higher, and 3) the collectors are normally not tilted for optimal gain in the winter. A system designed this way might produce nearly 100% of the hot water in the summer, but maybe only 50% in the winter -- maybe 75% year round.



In our design, a larger collector area is used. This is not expensive, since you are building the collector, and building it a few sqft larger is not a big expense in either materials or time. The collector is more steeply tilted for more optimal winter gain. The steep tilt is also important for the PEX version of the collector in that it help protect it from stagnation temperature damage, and better matches the system output to the demand year round.


Collector Alternatives

The PEX/Aluminum Collector:

The PEX Collector uses PEX tubing to carry the heat transfer fluid.
The fins that pick up solar heat and conduct it to the tubes are aluminum.
Care is taken to insure a good thermal bond between the PEX tubing and the aluminum fin.



The PEX collector is the least expensive, but it must be protected from high stagnation temperatures to avoid damage to the PEX (this is not insurmountable, and methods are discussed in the details). The collector is somewhat less efficient than the copper/aluminum collector described below, so it should be built a bit larger to make up for this.

Full construction details on the PEX collector...


The Copper/Aluminum Collector:

The Copper/Aluminum collector uses copper tubing to carry the heat transfer fluid.

The fins that pick up the solar heat and conduct it to the tubes are aluminum.

Great care is taken to insure a good thermal bond between the copper tubing and the aluminum fin.



Some simple soldering of risers to manifold is required in building this collector, but not much, and its easy soldering.



This collector is a more expensive than the PEX collector (about +$2 per sqft), but it does give about 13% better performance, is not subject to damage from high stagnation temperatures, and it is still less than 1/4 the price of commercial collectors.


I would characterize the copper/aluminum collector as a lower risk, higher performance choice that will cost a few dollars more to build. For most people, its probably the better choice.

Full construction details on the Copper/Aluminum collector...


My Systems Collector

The collector I built for my solar water heating system is of the PEX type, but is different in its size and aspect ratio than the PEX prototype collector.

The fact that I chose to build the PEX version should not be taken as a vote for the PEX over the copper version -- the choice had more to do with wanting to see how well the PEX works out, and to timing of the performance tests.

Full construction detials on this collector ...

A collector using CPVC instead of PEX-AL-PEX was also looked and rejected for the reasons described described here ...


Both of these collectors are low in cost at $4 and $6 per sqft, are relatively easy to build, perform well, can be made from locally available materials, and should have a long life. The savings compared to commercial collectors is of the order of $800 per collector ($600 on the collector + $150 of freight).



I've done small panel tests for both of these collector designs to compare performance to each other and to commercial collectors, and have also built full size prototypes of the PEX/aluminum Collector and the Copper/aluminum Collector with very detailed descriptions of the construction and some performance data. The full details on the PEX collector we are using on our system are also provided.



Any of these collectors will work fine with this system. Other collectors, including commercial collectors, could also be used. The major requirement would be that they work with a drain back system.




Storage System and Heat Exchanger



The storage tank is an EPDM lined, insulated plywood box framed with 2X4 lumber. The tank is vented to the atmosphere (non-pressurized). The water in the tank is used strictly to store heat -- it is not part of your potable water system. The heat in the tank is transferred to your incoming cold water before it goes to your current hot water tank. The tank is filled with plain water -- no antifreeze is used.



The transfer of heat from the storage tank to your incoming cold water is accomplished by immersing a large coil of plastic pipe in the storage tank. The incoming cold water passes through the immersed pipe coil and picks up heat from the hot water in the storage tank.



The pipe coil is so large that the water in the pipe coil itself will satisfy nearly all single hot water demands. Since the water in the coil has normally had a chance to heat up to the full storage tank temperature, the heat exchanger is 100% efficient in this mode. For very large demands, the pipe coil acts like a conventional heat exchanger.



Most drainback systems have a drain back tank that extracts heat from the collectors, and also have a pressurized solar heated water storage tank. This system basically combines these two functions into the one simple tank at a considerable cost saving.



Full Construction Details...


Plumbing

There is a bit of plumbing to connect the collectors to the tank, and a bit of plumbing to connect the tank to the house hot water system.

The collector plumbing consists of a line going from the submersible pump to the bottom of the collector, and a line back from the top of the collector to the top of the tank. If a non-submersible pump is used, than there is a line from the bottom of the tank to the pump mounted just outside the tank.

To plumb the solar heated water into the house hot water system, you basically break the cold water pipe that now goes to your existing hot water tank. You run a line from each end of the cut cold water pipe to each end of the heat exchanger coil in the solar water tank.

Full details here ...

Pump and Controls

The system uses a small submersible pump to pump water from the storage tank to the collector when the sun is out. When the pump shuts off, all of the water in the collector drains back to the storage tank for freeze protection. No antifreeze is used, and no heat exchanger is required on the collector side.



The pump is controlled by a differential controller. The controller has one temperature sensor in the collector and a 2nd sensor in the storage tank. When the collector sensor is hotter than the storage tank sensor by a set amount, the controller turns the pump on. When the collector temperature drops back down, the controller turns the pump off.



The controller's storage overheat feature is used to limit the storage tank temperature to about 140F. This makes life easier for the tank liner, pump, and heat exchanger pipe coil. The 140F maximum temperature also means that you may not need an anti-scald value, but this is a decision you need to make for your own family.

Full Details ...





Update Nov 11, 2008: Details on new pump...



Federal Rebates

You may want to take advantage of the federal 30% rebate on solar water heating systems. If you build the system exactly as described in these pages, you cannot legally claim the rebate -- this is dumb, but that's a whole other story. But, if you substitute commercial collectors for the homemade collectors, you will meet the requirement that the collectors be certified under the SRCC OG-100 program. If you do this then the full cost of the system becomes eligible for the 30% rebate. Just make sure that the commercial collectors you buy have been certified by the SRCC.


More information on the various types of solar water heating systems and other plans for DIY solar water heating systems ...


Kristy and the collector.

Gary September 12, 2008