Practically off the grid!

In an effort to minimize space requirements in my kitchen I chose to avoid installing a common stove top and use a smaller portable hot plate for my stove top cooking. Since portable hot plates do not have a huge share of the market there are not many options out there. I started out by purchasing a basic hot electrical element hot plate and have now switched to an induction hot plate based on the claims that they are more efficient. Well, today I decided to finally put them to the test and find out the most efficient hot plate for myself.

The competitors are the Salton hot plate ($30) and the Earthchef Induction hot plate by BergHOFF ($200 on sale). The test I have chosen is to use both hot plates to heat 2 litres of water by the same amount and measure the differences in power consumption. To measure the power consumption I will be using my electronic energy meter and to measure the temperature I have a standard alcohol thermometer.

 

 

The Salton hot plate was the first to be tested.  The water temperature at the start was 12 C and it was heated to 90 C in 20 minutes.  The readings on my energy meter showed that this task consumed 0.32 kWh with a peak power draw of 1471 W. 

Next, I repeated the procedure for the Earthchef Induction hot plate.  Again, the water temperature at the start was 12 C and it was heated to 90 C but this time the task only took 12 minutes.   The Earthchef hot plate used 0.25kWh and had a peak power draw of 1322 W.  

 

 

These numbers already show that the induction hot plate is more efficient but I wondered how much more efficient.  To approximate the efficiency of each device I must know the amount of energy needed to heat 2 litres of water from 12 C to 90 C. To calculate this energy total I can use the equation Q=mcdT where Q is the energy used in kJ, m is the mass in kg, c is the specific heat capacity of water 4.186 kJ/kgK and dT is the change in temperature in degrees Kelvin.  The math looks like this:

Q=(2kg)(4.186kJ/kgK)(78K)

Q=653.016 kJ

1 kWh is equal to 3600 kJ so I can use this ratio to make the conversion to kWh.

Q=653.016kJ / (3600kJ/kWh) 

Q=0.1814 kWh

So the heating of 2 litres of water from 12 C to 90 C requires about 0.1814 kWh of energy.  Yes there are other things to account for like the heating of the pot itself and the energy losses from the hot water as it is being heated but in the interest of simplicity I’m going to neglect those factors.  Sadly, the Salton’s slow cooking time only worsens its overall efficiency because this added time means more standby energy losses.  At least the controlled variables have been consistent throughout the experiment.

The Salton hot plate used 0.32 kWh to heat the water so that means 0.1386 kWh was wasted.  The appliance was therefore only 57% efficient.  The induction hot plate with a power consumption of 0.25 kWh wasted only 0.0686 kWh.  Its efficiency was a much improved 72%.  I am glad the induction hot plate came out on top because it was much more expensive but I am a bit surprised by how much energy is wasted by both units.  I could have reduced this energy loss by using a lid for the pot but I needed to leave the lid off to take temperature readings.  An insulated pot would also perform better so that makes me wonder how an insulated crock pot would perform.  Perhaps that is a test for another day.

 

 

There are a couple of other factors to consider if you are thinking about purchasing one of these hot plates.  Induction models are louder because they have a cooling fan under the hot plate.  The trade off is the low temperature cooking surface.  The induction technology uses magnetic energy to heat the cookware so the cooking surface is actually safe to touch.  This is a great feature that makes cleaning up spills really easy and makes cooking safer.  The induction surface is flat and easy to clean whereas the electrical elements are complex and any food that inevitably spills will make a big smoky mess.  My particular induction cooktop also allows me to set the exact temperature of the surface.  This is great for simmering for long periods and cooking finnicky dishes.  The induction hot plate is the clear winner in many areas of performance.

This experiment was decisive in determining that the Earthchef induction hot plate was the more efficient option and its other features make cooking with it a breeze.  If you are heading off the grid and want some kind of electrical cooking device this may be the best choice for you however I would also consider a well insulated crock pot and of course do not over look the potential of solar cooking.  That should after all be our first choice if we wish to live a sustainable lifestyle.

 

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.