Solar Water Tanks

Solar water tanks are used in solar hot water systems to act as  buffer tanks.  When the sun is shinning, the water will be heated in the storage tank for later use, most commonly in the evening.   Most solar water tanks contain a heat exchanger to seperate the potable water from the solar heating solution (Water/Glycol), and have a great insulation value that can retain the heat for day. 

There are 3 kinds of solar water tank systems used in North America:

• Solar Pre-Heat Tank
• Solar Combo Electric Tank
• Solar Dual Heat Exchanger Tank

When sizing a hot water storage tank you need to consider two important variable.  First, how much draw is used per person in the house?  An average house hold will consume 20-30 gallons per person.  This includes shower & bath water, dishes, and laundry.  So a family of two will  normally require a 40 gallon storage tank at a minimum.   Secondly, for each gallon of hot water you need the same number of square feet of collector area.  It is very important that you don't exceed this number or you will find that in peak summer months your solar system may reach extreme temperature.  It is better to have a system that is under powered than over powered.  Our TZ58/1800-30R solar evacuated tubes have a gross area of 40 ft² aso for each collector you will need 40 gallons of storage.

Comparison Chart

Contender SSC	Contender SE	Contender SB
Pre Heat only with 2nd tank	Replaces existing tank with combo solar/elect	Used for dual solar applications

 

The 1st and most common system is a Solar pre-heat tank.  This type of solar heating system is used in homes with an existing water heater (electric, oil, gas, tank-less).  The pre heat tank stores the solar heated hot water which is then drawn into the main tank when the hot water faucets are open.  This type of system always has a can use an external heat exchanger or an internal heat exchnager (see conversion tip below).    The pre heat tank heats the other wise cold ground water before it enters the normal water tank.  In this way ground or main line water of approx 35 F can be preheated to say 120 + degrees before entering the primary heating tank.  If the heating tank is set to 130 degrees it only needs to raise the water 10 degrees instead of the 95 degrees it would have otherwise had to with out the solar pre heat tank.

The second type of solar water heating system uses a combo tank with solar and Electric.  The combo tank contains a built in heat exchange on the bottom of the tank and also contains one upper element.   The solar hot water is circulated the solar loop and in the tank and heats the water in the tank.  The top element will then add to, and top up, the tanks temperature to the desired setting.  This type of system is great for new homes or replacement tanks .  It is important to note that very few solar tanks found in North America meet UL or CSA certification as a combo tank, as such most European or Asian manufactured solar tanks can not be used with potable (drinking water).  Northern Lights Solar Solutions tanks are 100% UL listed and can be used in any solar application in US or Canada.

 

The final type of tank is suited for dual solar heating applicaitons and uses two internal heat exchangers.   These tanks contain a bottom and top heat exchanger whereby the solar loop heats the tank from the bottom,  The upper loop is found in the hottest part of the system and contains the second loop.  Water surround the upper and lower tanks and acts as a heat buffer for storage until the upper coil has heating demand.  This type of tank is designed for both heating applications and/or domstic hot water.  Multiple tanks can be used to build a large scale heating project.  These tanks are used primarily with glycol loops for the solar and a separate glycol loop for the heating.  The larger the tank size, the more storage capacity.

 

 

 

How to make an inexpensive Solar Hot Water Tank with a External Heat Exchanger

 

The Contender Series of solar tanks are an excellent choice for most solar applications, however for those on a tight budget, there is a very affordable way to create a solar storage tank using either a new or used electric hot water tank and an external heat exchanger.  Electric tanks are perfect for a solar conversion as they have all the ports you will need along with existing safety features such as a thermal pressure release valve and a build in drain valve.  A average hot water tank can be purchased for $300-$500 and converted in minutes for under $10 in parts plus an external heat exchanger.   To convert a hot water tank we simply remover the two heater elements which are threaded into the tank (1" thread). The top element port will become the supply for the hot water and the bottom heater element will become the draw for the colder water from the bottom portion of the tank.

In smaller systems you can allow thermalsiphon to move the water from the bottom of the tank to the top portion.   Thermosiphon is the natural tendency of hot water to rise as it weighs less.  As the heat exchanger heats up the potable water, the hot water will rise to the top port and draw in the colder water from the bottom port.  On larger systems especially commercial, a secondary pump can be used on the storage side to circulat the water in the tank.

 Below are 4 easy steps to covert an electric hot water tank.  In this example we have used a heat exchanger as the winter temperatures get very cold and a 50%/50% water and glycol mixture was used in the collectors.  However if you are in a milder climate with out hard freezing, water can be used and you can avoid using a heat exchanger, connecting directly to the water tank. 

Making a Solar Storage Tank

 

Image		Description
		purchase a new or used electric hot water tank with two heater elements.  There will be two covers over each element port, that once removed you will find the hot water elements which screw out.
		remove heater element and thread in 1" mpt to 3/4" mpt reducer.  These will become the return and supply lines.  (top will be supply  & bottom return)
		connect 3/4" mpt to copper or PEX piping according to the size of pipe to be used (recommend 1/2" copper or 3/4" PEX).  Depending on the size and type of pipe you will need to buy a 3/4" ftp connector fittings, found at any hardware store.
		connect the supply and return to from the solar collector to the heat exchanger (or directly to your ports if no heat exchanger is used). Add insulation to piping and heat exchanger. That's It!