Passive Solar Heating: Solar Thermal Energy for Hot Water

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Passive Solar Heating: Solar Thermal Energy for Hot Water: DIY Heating Element

DIY Heating Element

Passive Solar Heating: Solar Thermal Energy for Hot Water: Passive Solar Water SystemAlthough it isn’t the largest bill in the home, hot water accounts for 10% of the energy consumption in the typical home.  This means that in just a year’s time, you could be spending up to $600 a year on hot water, which doesn’t even include the water you are using!
Wouldn’t you like to get a quick return on your energy efficiency dollar?  Passive solar heating, Semi-Passive, or Active Solar Heating may be the solution.  Using solar thermal energy for a hot water heater is a simple concept.  Let’s dive in shall we?
The Sun hits us with an almost unlimited supply of radiant energy, and everyone wants to instantly turn that into electricity using photo-voltaics.  Not only is this method expensive, but it is low in efficiency.  You have to buy panels, batteries, chargers, inverters, and tie all your normal services on it, such as a hot water heater.
If you have known me for very long, you would know that I am an advocate for photo-voltaics for small loading situations, but not to run large loads like a furnace or hot water heater.  Even if you have natural gas to your home, the resiliency of a DIY, functional, homebuilt system is unparalleled, especially for hot water.  A simple system is all you need, and much of it can be done with little to know pumping of your water… i.e. passive.
Let’s go through a few ideas for hot water using the abundance of solar thermal energy:
The first idea is an active system with a tank, pump, and heat exchanger
Active Solar Heating: Solar Thermal Energy for Hot Water

Active Solar Heating: Solar Thermal Energy for Hot Water

This means that it will require power to function.  You keep a loop pump running to circulate water through a simple heat exchanger and back to a tank.  The advantage of this type of system is that it takes less space and will give a higher BTU return.  This system will force heat the water, causing a larger volume of heating.  Another advantage is that you don’t have to properly design where the water goes in and out to get the proper flow for the heat exchanger, and the pump doesn’t have to be too incredibly large.
The next active solar water heating system is a system using glycol
This heat exchanger is efficient, allows for hotter water, and it ties in with your water heater quite well.  As a product of this, you will use less hot water to do things like take a shower.  A solar heat exchanger has glycol pumped through it.  Then the glycol goes through another secondary heat exchanger which is in the hot water heater and will heat up the water in the tank.
A common passive system will use just a tank, and heat exchanger.
Passive Solar Heating: Solar Thermal Energy for Hot Water: Passive System on Roof

Passive Solar Heating: Solar Thermal Energy for Hot Water: Passive System on Roof

The passive system that I am talking about is a pretty simple concept and design, but slow in heating and very particular in how the water line is laid out.  If you have a tank outside beside your heat exchanger, you will plumb the bottom of the tank to the bottom of the heat exchanger and then the return line will run from the top of the heat exchanger to the top of the tank.  As the water heats up in the exchanger, the density of the water will go down causing it to travel up and the pressure of the tank to overcome the resistance of the exchanger.  This will cause a natural flow through the heat exchanger and it is called a “thermal siphon.”  This design takes a long time to heat the tank to a usable temperature, but requires no energy, so it will work when the power is down.  There is no pump failure either… because there is NO pump.
Passive Solar Heating: Solar Thermal Energy for Hot Water: Left over Parts
Be careful when running the lines between the tank and exchanger, though because any droop or sag in the lines will break the thermal siphoning effect.
My favorite system is both an active and passive solar heater
Dual Active - Passive Solar Heating: Solar Thermal Energy for Hot Water

Dual Active – Passive Solar Heating: Solar Thermal Energy for Hot Water

Not only this, but uses a concept called “thermal mass.”  The above system is connected except it is connected to an enormous 1000 gallon tank.  There is a T-connection at the tank and heat exchanger to create a parallel loop with a small recirculating pump on it.
The system will work on minimal power to provide forced heating of the water and recirculate it through the tank.  If the power to the pump goes out, or the pump fails (since we will have it on backup power right?) you will be able to valve the active loop off and use the passive loop.
Passive Solar Heating: Solar Thermal Energy for Hot Water: DIY Heating Element

DIY Heating Element

The tank itself is so large that even at a lower temperature, the sheer volume allows for general use of the hot water.  At that point you don’t need to mix as much cold to bring the temperature down to a comfortable level.  I recommend using black material on the tank to prevent algae and to add thermal absorption properties.
The thermal mass will also keep a decent water temperature… day or night, because it takes so long for the mass to be heated or cooled.
If you use system water pressure and/or rain runoff to fill the tank, you will have a large reserve water supply available, even when supply is generally not.
The last option is the one that I prefer due to the water reserve, the large thermal mass, and the resiliency of the system.  Which system have you seen or would you use?
Solar Heating: Solar Thermal Energy for Hot Water

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