Practically off the grid!

My philosophy with solar air heating is to go big or don’t bother.  One reason for this is that solar air heaters are so cheap to build that increasing the collector size really does not add a huge amount to the cost of construction.  Also, since the materials you have available for a DIY heater may not give you the most efficient heater, it won’t really pay off to build an undersized heater anyway.  If you are going to buy into the same philosophy with your solar air heater project then you will want to build it right the first time.  To aid you in your design process, I have compiled a list of research experiments on the subject of solar air heating.  I am not sure if a comprehensive resource exists on the subject but if you piece together the bits of knowledge presented by each of these papers and incorporate the best techniques in your design you will be bound for success.  If you know of some more valuable information out there that I have yet to find please send me a link to it and I will add it to the list.

Solar Air Heaters – An Application Guide

This paper offers a simple comparison of unglazed, transpired and glazed collectors.  It includes some basic descriptions to help you understand the basic differences between each design strategy but does not have a lot of experimental data to share.

Analysis of High Efficiency Solar Air Heater for Cold Climates

This paper is a well documented study of a collector using porous material for the absorber.  There are a lot of equations and graphs presented that back up the theories presented.  

Experimental Investigation of Solar Air Heater with Free and Fixed Fins

This paper may change some of your perceptions about collector efficiency by offering a well recorded comparison of a flat plate, fixed finned and free finned solar air heater.  You might think it would be best to keep all of the air channels open to keep the air flowing through quickly however this does not actually increase the collector efficiency.  

Numerical Simulation Study on Transpired Solar Air Collector

This is a neat mathematical simulation of a transpired collector.  This type of design may be the best fit for you if you are planning on using your solar air heater to preheat outdoor air on it’s way to your air exchanger.

Solar Air Systems:  A Design Handbook

This is perhaps the most comprehensive resource on solar air heaters that fully integrate their air loops into the building structure through walls and floors.  Enjoy this free Google Books preview while it is still available.

Thermal Performance of Wire-Mesh Roughened Solar Air Heater

If your collector uses a flat absorber plate you will need to create turbulence behind the absorber to transfer as much heat as possible from that hot absorber and increase your collector efficiency.  One method to do this is to fasten wire-mesh behind the absorber plate.  This is a simple experiment comparing the effectiveness of different sizes of wire mesh.

Comparative Analysis of Active and Passive Solar Heating Systems With Transparent Insulation

This paper compares data from six different collectors in France that use either a passive or active design.  Transparent insulation was used in some cases and was shown to increase the collector efficiencies.

Simulation of Solar Heating at a Constant Temperature

This report describes an air heating system that incorporates a large heat storage mass and variable speed fan to manage the temperature swings that come with solar heating and deliver heated air at more consistent temperatures.

Air-Type Solar Collectors for Agricultural and Residential Use

This is a comparison between collectors with air flow on top of the absorber, both sides of the absorber and behind the absorber.  Which design do you think came out on top?

Thermal Performance of Solar Air Heater by Using Shot Peened Absorber Plate

This paper shows how an increase in roughness of the absorber plate can lead to a significant increase in collector efficiency.

How to Build a High-Efficiency Air-Type Solar Space Heating Collector

This document offers detailed plans for a collector that uses black polyester felt for the absorber.  Air is forced from one side of the felt to the other while it passes through the collector so all of the air must make contact with the warm surface.  In theory, this should lead to higher efficiencies and according to Bill Kreamer it operates with over 72% efficiency.

In your transition towards off the grid living you will definitely be concerned with your electrical power consumption at some point.  When it comes time for you to cut back your demand for electrical power it will be helpful to isolate each appliance you have to measure its power consumption.  A plug in electronic energy meter is a simple piece of technology that can make this task very easy.

 

I found this Blue Planet Electronic Energy Meter for $25 at Canadian Tire.  It has a three prong plug on the back that you plug into a standard electrical outlet in your house.  Then you plug in any appliance and begin measuring the power consumed by that appliance.  Many appliances will have a power rating sticker of some kind but those ratings could change with the age of the appliance (especially refrigerators) so it is always best to just measure the power consumption yourself.  This device is also useful to measure phantom loads appliances draw even when they are turned off.  Simply plug any appliance into the meter, walk away and let the meter take the stats for you.  When you return hours or days later you can flip through the accumulated totals and see if that particular appliance is to blame for your high energy demands.

 

 

In order to give some examples of the data this meter provides, I have plugged my laptop into the meter for a few hours.  Here are some photos of the display screens:

As you can see the displays include readings for the outlet voltage, instantaneous power use, maximum power use, instantaneous current, maximum current, price of electricity per kWh, accumulated cost for power use and time of use.  That is plenty of information to assess the power demand of any appliance.  The time reading at the bottom shows that my laptop was plugged in to the meter for just over two hours.  In that time 0.01kWh of electricity was used for a total cost of $0.001.  You can see on display screen 2 that I have entered my local electricity cost of 11.24 cents/kWh.  The demand of the laptop varies drawing around 0.16 A or 7W.  These are instantaneous measurements so if the appliance you are measuring draws an irregular amount of power and you want to determine an average power consumption you need to divide the total number of kWh used by the time of power use.  

The increments of measurements are fairly large so this electronic energy meter is better suited to measure larger appliances.  Because my laptop only draws around 7W it took 2 hours of laptop use to accumulate any significant totals on the meter.  These totals will accumulate much more quickly when you use the meter to measure the power consumption of hot plates, microwaves, blenders and other more demanding appliances that use 100W to 2000W.  

I have found the meter to be a functional and convenient tool to satisfy my curiosity about the power demands of any appliance.  In fact, this is the meter I used to compare the power consumption of my old and high efficiency refrigerator.  It was that initial power reading of my old refrigerator that motivated the entire high efficiency refrigerator project.  I knew the old refrigerator was old, loud and probably inefficient but when the meter showed me that it consumed between 5 and 6 kWh a day I knew a change was necessary.  Pick up one of these meters and start plugging it in behind all of your old appliances.  Whether you are preparing for high efficient off grid living or just satisfying your curiosity you may be in for a surprise and a change of lifestyle.

The recent modifications to my solar air heater required some changes to the vents in the front entry.  While I am always delighted to see the solar powered fan spinning away I wanted something to dampen the sound a bit, prevent any backdrafting and make some use of all of that wall space.  

The solution I decided on was to build a coat rack with enclosed inlet and outlet vents for the air heater.  I learned from some trial and error that the vents on the top of the shelves had to be positioned horizontally so that cool air did not flow out of the vents when the heater was not operating.  With the vents positioned like so, the cool more dense air just sits below the vent waiting to be pushed by the fan when the Sun comes out. 

 

The large square openings on the left and right sides allow for convenient access to the fan and duct space behind the unit in case I ever need to make any small changes.  The photo below shows the finished coat rack with the square openings now covered with cork board.  I’ll find something fun to stick on there soon.

The rack has been installed for a few days now and I am pretty happy with everything so far.  It allows me to make better use of the space in the front entry and it also works to dampen the fan noise and prevent backdrafting as required.  One downside may be that it does impede air flow.  I kept the openings as large as possible to minimize the additional air flow resistance but I think the unit has slowed things down slightly.  With a rating of 500CFM my fan for this solar air heater is still a bit oversized so I am not worried about sacrificing much of the performance.  I’ll continue to make observations and make any changes if needed in the future but as for now I am excited to be finished the project and plan to enjoy the solar air heater in its completed state for some time.