Sorry everyone - I know I shouldn't trust my memory anymore. I just checked the fridge again and its using 130W. Looked like about 300W for a few seconds when it started up. I have a proper powermate meter Lance - much more reliable than my memory!
Couldn't find the fridge specs but looks more like 300l than 160l.
Anyone got a small/bar fridge that they've measured the power level on ? I still don't think its feasible to do what these friends want to do - ie run a small fridge directly off a 100W solar panel via inverter. Besides the startup problem, what would happen if it got cloudy ? Would the inverter shut down or possibly supply low V ac to the fridge motor and cause damage ? I know the solution is to insert a decent size battery to smooth out the energy requirements as the fridge turns on and off, but then its all getting a bit complicated for a portable demo system The direct dc fridge sounds much better. (Sorry for hijacking the thread dymonite).

My fridge is 200L and quite old - it is rated 4 stars on the old system so probably 2 star under the new rating system. Measured at 150 Watts (compliance plate says 130W - so it must be getting old). Peak load was about 1kW. It used 0.9kWh per day when measured - but that was in winter so probably it would be higher when averaged out over the year.

I measured an old fridge owned by a family member - not star rated, about 350 litre but it used about 3kWh per day (also in winter). I recommended replacement with a new more efficient fridge - should pay for itself within 7 years.

Benny, startup on your fridge is probably 1kW. Once the motor is running, it drops a lot. Mine is about 100W. Fridge consumes about 1kWh per day. Freezer consumes about 2kWh per day, running nearly all the time, so about 85W. I should replace both with a single more efficient unit, but that's more money.

Benny

I seriously doubt your fridge uses 1kW while running, I bet you are using a Jaycar or similar cheapy energy meter to measure it, those things are hopelessly way off for anything other than a purely resistive load. If you are using a better meter, you might have checked it when the defrost heaters had kicked in, but even then they are usually only a few hundred watts. 1kWh a day is a fair bit for such a small fridge, my 420 litre LG unit measured out at 0.98kWh a day during a week of monitoring last summer. Admittedly it doesn't get opened anywhere near as often as many fridges, but sounds like you don't have a big family, with a 160 litre fridge.

Hi dymonite. I too appreciate your desire to spread the "theoretical" word on some basic physics calculations around so that others can get a basic appreciation of just what energy/power etc mean. I've seen lots of your posts here and on the envirotalk forum and I'm of a similar leaning. I think some people here are being a bit harsh on what was your original post - just to put some basic physics out and hopefully help others to be able to use this to help make decisions about maybe non-standard uses for solar PV/heating.
A classic example is the other post here with the guy wanting to consider heating his pool with an ET array. It would be very hard to get definitive quotes from a retailer of ET systems about what size/how many tubes would be needed and I'm sure he would like a second opinion on the feasibility - which I presume is why he asked here.
Someone else wants to know whether they could store enough heat underground during summer (from ET water heater) to be able to use this heat to warm the house during winter - I'm doing a rough simulation of that just to see if it (theoretically) looks feasible. Some basic physics on heat capacity and heat conduction is all thats needed.
I agree with others that its great to have real world figures from operating systems, but there's also a place for some physics/simulations when its a non-standard use. My biggest gripe is seeing how often people get kW and kWh confused so spreading the word on some basic physics is great.
Still I'm sure everyone here is on the same page when it comes to the necessity for more sustainable energy use. Great to see the contributions from everyone.

Oh but one more comment on your original post - you stated that a fridge only uses about 60W of power.. Are you sure thats correct ? Mine uses nearly 1kW when operating and its only about 160l. Over a day it only uses about 1kWhr so its pretty good on consumption. I recently tried to find the power consumption data for fridges as someone I know wanted to run a small fridge from a 100W solar panel via an inverter and I had to explain why this wasn't feasible. But I couldn't find any power ratings for fridges - only energy ratings in kWh/year. Lots of comments though that they often draw a big current at startup so making direct drive from panels/inverters even harder. Looks like a small esky type camping fridge would be OK, 24V/4A but >$1000.

Sunshine,

I am glad that your company had the integrity to provide a real world figure rather than am optimistic one. I am not so sure that all clients get the same treatment.

The economics of solar are advantaged in places like Southern and coastal Queensland. You receive good sunshine but with low household heating/cooling requirements. With HVAC making up to half of a household's energy needs this greatly reduces the requirement for a large array.

The equation changes significantly down here in the temperate/cool temperate parts of Australia. There is 15-30% less sunshine and quite high heating requirements (and hence energy consumption).

Here is map of insolation around Australia:

http://www.bom.gov.au/cgi-bin/climate/cgi_bin_scripts/solar-radiation.cgi

dymonite69,

Depends upon which figures you use to start with.
All the solar company's in Brisbane advised me the expected return for a 1kW system is 4.2kWh per day.
So in this instance the real world performance is better.
The solar installer said with the Schott panels I bought, the yield will be considerably better, which turned out to be true.
No different to oil or coal.
To get coal or oil ready to be used as an energy product, takes alot of energy to be mined and shipped around the world. Whilst solar energy doesn't suffer from such losses.

Sunshine,

I tend to find that most people expect they can get more out of a solar system than what is actually possible. The theory confirms you already need a fairly large collector to drive a number of applications e.g. generate steam, solar cooling, run a ducted AC system, hydronic floor heating, charge an electric vehicle. The real world confirms that the situation is even worse.

dymonite69,
There is no point quoting large reams of theoritical equations if they are way off the mark in the real world.
People want to know what a system will perform to, not what a system will produce in a lab experiment.
Otherwise you will giving people false expectations if they are thinking of installing solar energy systems.
FYI, the rated Brisbane average return for solar PV is 4.2kWh for a 1kW system, so that makes it 12.6kWh for a 3kW system.
I am producing a 14.8kWh/day over a year, so that's a bonus.

dymonite69, looks like you had the specific heat capacity of steam which is 0.002MJ/kg-K

Sorry. You are right. Order of magnitude 3x wrong. 2.26 MegaJ/kg. Not much steam after all.

dymonite69,
Sorry for insisting, but this is really important as many people don't know about it.
The heat of evaporation of water is very large. The energy required to evaporate 1 kg of water at 100 degrees to form steam is actually more than 6 times the amount of energy required to heat 1 kg of water from 15 to 100 degrees.

This means that with your example of 25 MJ energy you can just convert about 10 litres of water into steam, not 72 litres.

I don't know where you got your numbers from, but it takes 2.26 MJ/kg to evaporate water at 100 degrees, not 0.002MJ/litre as you mentioned.

dymonite69, you still haven't got the litres of water into steam correct. It appears you have forgotten the latent heat of vaporization.

Energy to convert 1kg of ice to 1kg of water = 334kJ
Energy to heat 1kg of liquid water by 1C = 4.18kJ
Energy to convert 1kg of water at 100C to steam at 100C = 2260kJ
Energy to heat 1kg of steam by 1C = 2.08kJ

It takes almost as much energy to melt 1kg of ice, as it take to heat water from 0C to 100C.
It takes far more to convert it to steam.

So the calculation should have been
25 MJ / (0.004 / (100 degrees - 15 degrees) + 2.26) = 9.6l of water into steam

Your use of basic physics to put some sense into solar comparisons is appreciated.

Thank you for your replies. The intention of the post was not to discuss all the various issues raised but so that the general readership can appreciate the use of solar energy for the particular scale of the project that they are considering. But I will answer the points in order.

Sunshine said,

"All good in theory, however real world figures are different."

"For my solar PV system, the 3 months of summer produced 1231kWh at 13.68 kWh per day, whilst winter produced 1248kWh at 13.56 kWh per day."

In reply:

Yes. All my figures are optimistic ones. Theoretically you should produce 14 kwhr (winter) and 28 kWhr (summer) with a 3kW system such as yours. There are various reasons people can't obtain this in the real world e.g. non-ideal orientation, shading, high temps, poor installation etc etc

Sun2stream said:

"I would like to add a few things because what you say (I suppose with the best intentions) is actually used by renewable energy sceptics to make the point that renewable energy will never be able to supply the energy we need."

In reply:

The example I gave with the various energy densities of fossil fuels was for comparison sake. It was not to support or refute the use of renewables. What you say is true about the inefficiency of coal generating power plants (30%). But there is nothing to stop someone burning a nugget of coal in their wood heater to improve on that.

Ignoring industry, transport is a vast consumer in the energy sector. It has rapidly expanded due to the availability of cheap fossil fuels. Will solar meet this need? Electric cars are still a novelty requiring large batteries to power them and either suffer from modest performance or lack of storage space (e.g. Prius vs Tesla). A significant expansion on a domestic solar array would be required to charge a car for daily commuting. Should we abandon all thoughts of private domestic travel if we are to achieve a more sustainable future?

The economics of fossil fuel versus renewables warrants a separate discussion (factoring in things like mining costs, maintenance, interest rates, subsidies etc etc). As you say $/kW or $/kWhr for the consumer (and for the lifecycle of the installation) is the more important figure. It will require the current price of $4-8/kW to reach $1/kW to become cost-competitive.

Sun2Stream said:

A few corrections:
"'Water heated = 25 MJ / 0.004 / (100 degrees - 15 degrees) = 73L water for steam/day'
This is not the conversion into steam! it is just heating water to 100 degrees. Phase change from liquid water into steam takes much more energy."

In reply:

I stand corrected. There is an additional 0.002 MJ/litre to take 100 degree water and convert it to steam. Instead of 73L water for steam it should be reduced to 72L water to steam.

Rockabye (from Rockhampton, QLD) said:

"My 1kW system can provide all household electrical energy consumption for two energy savvy people"

In reply:

South of the Queensland border home heating becomes the predominant consumer of household energy. We may not be using bar radiators to do this but even highly efficient heat pumps (RCAC) still consume significant electricity. Most people with domestic PV installations avoid electrical forms of heating for this reason - preferring gas or wood. However, neither of these are sustainable options. All things considered, to convert to pure solar electricity would require far bigger PV arrays then they currently have now. For a person living Hobart or Melbourne this would be a significant expansion.

Have to agree with most of the replies re real world data vs calculated output. Here in Rockhampton my best performance has been around the spring equinox. Overall there is about a 15-20% difference between worst month and best month since installation in June.

It is good to use both models to calculate performance but there are so many installation variables that only one method is valid for a given installation type at a known location. For example my friends 1kW Nth facing thin film array located just 10km away is producing about 15% more than my 1kW NW facing monocrystal array.

Addressing the title, what can solar energy do? My 1kW system can provide all household electrical energy consumption for two energy savvy people. I would need an extra 1kW array if I wanted to have more non essential electrical loads but my roof could easily accommodate another 2kW if required. That's the bottom line.

dynamite,
You have started a really interesting discussion.

I would like to add a few things because what you say (I suppose with the best intentions) is actually used by renewable energy sceptics to make the point that renewable energy will never be able to supply the energy we need.

You say that 1 kg of coal has 30 MJ of energy.
A bit later you say that PV has an efficiency of just 20%. It is actually not really 20% just about 16% to 18% on most domestic systems.

And this percentage looks so bad. Is it really? The efficiency of coal in coal fired power stations is also around about 20% if the coal is used to create steam which drives turbines to create electricity. And this does not even include all the effort (=energy) of coal mining, transport, storage and dealing with the waste. In the case of 'carbon capture and storage' the efficiency will drop even much further.

The high energy density petrol you mention has just an efficiency of less than 15% if it is used to power a car. And again it does not include the energy required for production, treatment (distillation, cracking), transport and distribution of the fuel.

If you compare the size of a the area for coal mining, burning and waste treatment/storage to the size of a solar power plant, solar is actually the much more efficient (in regard of required area ) form of energy. http://americansolareconomy.blogspot.com/2009/01/solar-vs-coal-land-area-comparison.html

Efficiency is one of these tricky words which changes it's meaning depending on the context. But most people are not aware of it.
So even if the efficiency of a coal fired power plant may be the same percentage number of the efficiency of a PV module, it is not the same thing. The coal fired power plant needs permanently fresh fuel to be mined in order to run the plant. The solar system does not need fuel to be mined. The solar system is harvesting solar energy.

A few corrections:
PV does not have a 80% energy loss. According to your numbers it has 20% efficiency. This is quite different.

'Water heated = 25 MJ / 0.004 / (100 degrees - 15 degrees) = 73L water for steam/day'
This is not the conversion into steam! it is just heating water to 100 degrees. Phase change from liquid water into steam takes much more energy.

'... anytime you convert energy, losses will be incurred.'
Mostly true, with the exception that eventually all energy is converted into heat energy. And conversions into heat energy can be 100% efficient.

And of course nobody in his right mind would use electricity generated from PV to run an electric bar radiator. This may change in the future if we can generate electricity from solar energy at lower cost than from fossil fuels.

The efficiency of PV systems is not so much the issue. It is rather the cost in $ per kW and $ per kWh. Efficiency is only important if you want to maximise the energy output of PV on a limited area, like on a domestic roof.

dymonite69,

All good in theory, however real world figures are different.
I agree with Greenvalue on this one.

For my solar PV system, the 3 months of summer produced 1231kWh at 13.68 kWh per day, whilst winter produced 1248kWh at 13.56 kWh per day. Basically just about the same.
I live in Brisbane, so winter is just about sunny all the time, whilst summer has lots of cloud due to storm and humidity.
The 3 months of spring produced 1465kWh at 16.1 kWh per day. Spring here is still sunny, with minimal cloud, but days are longer, hence greater yield.
So theory is fine, it doesn't take local weather conditions which can dictate solar performance more than insolation variations.

Then, as we use very little power in winter (no aircon or heating required), we export around 80% of what we produce, but in summer, we export only around 50% of what we produce.
So winter, our net yield (export) is much better than in summer, eventhough insolation in winter is 50% less.

Another interesting thing I have noticed is peak power output in summer is exactly the same as in winter, eventhough summer insolation is 50% higher, this is negated by the higher panel temperature, and greater power loss.
So, eventhough winter insolation may be 50% less than in summer, it does not effect the overall yield, as my above figures show.
Hence, theory is interesting, but it doesn't always exactly hold up that well in real world performance figures.

Lismore BOM data:

http://www.bom.gov.au/jsp/ncc/cdio/cvg/av?p_stn_num=058037&p_prim_element_index=31&p_display_type=statGraph&period_of_avg=ALL&normals_years=allYearOfData&staticPage=

Annual average: 18.2 MJ/m2
June: 11.1 MJ/m2 (40% less)
December: 24.2 MJ/m2 (30% more)

Or choose your own city:

http://www.bom.gov.au/climate/averages/index.shtml

Efficiency ratings quoted is at optimal conditions. Higher temperatures will degrade it about 1/3 % per degree.

Why are you quoting the winter output as 50% less? Is this under the assumption that you have more overcast days in winter?

Because then up here (the beautiful North Coast of NSW) that's not true. Winter is our driest and sunniest time of the year, just a little cooler (at ~20 deg average)...

I have also read that pv panels are more effective in cooler temperatures; that's why countries in Europe (specially Germany) are installing so many larger pv roof areas and facades all across the country: they work very well even during their shorter days in winter because it's also colder. In summer heat you have to make sure the panels are well back ventilated.

A common question on this forum takes the form of, "I would like to know how feasible it is to use solar energy to heat my xxx or operate my yyyy"

It is often quoted that there is enough solar energy falling on the earth to easily supply our world's energy needs many times over.

However, there is a big catch: Solar energy has a very low energy density and you need vast collector areas to harness all of it.

For comparison burning 1kg of coal releases about 30MJ of energy. *

On the other hand if you took the average insolation in Sydney, Australia you would need to collect an entire day's worth of sunshine over a 2m2 area to produce the same energy.

This is the yearly average. In winter it will be roughly 50% less and in summer it will be 50% more e.g. vary from 8 MJ to 24MJ

The next part is to convert that solar energy into a form of energy that it is useful.

Solar HWS convert solar radiation into pure heat. Photovoltaics turn it into electricity.

As a general rule, anytime you convert energy, losses will be incurred. Solar hot water heaters suffer between 20-50% energy loss. It is even worse with photovoltaics which lose 80%.

Assuming the aforementioned 2m2 collecting area, a solar HWS might generate about 25MJ energy and photovoltaic about 6MJ (or 1.8 kWh) electricity. Incidentally a 2m2 collector is around the aperture area ** of one standard solar HWS collector and the same area as a 0.3 kW PV system.

So much water can 25MJ of energy heat?

It requires 0.004 MJ to heat 1 litre one degree Celsius.

Let's assume the water is already 15 degrees.

To heat it to 60 degrees for a hot water service:

Water heated = 25 MJ / 0.004 / (60 degrees - 15 degrees) = 138 L hot water/day.

To heat it to 100 degrees for steam:

Water heated = 25 MJ / 0.004 / (100 degrees - 15 degrees) = 73L water for steam/day

So what about running electrical appliances off photovoltaics.

There are two considerations. One is the total energy consumed over the course of a day (MJ or kWh) and the second is the rate (J/s or watts) at which it is consumed.

The latter is not so much of a problem if you draw auxiliary energy off the grid or remove it from previously stored energy from batteries.

To run an appliance in realtime purely by solar energy requires that the demand of the appliance does not exceed the instantaneous output of your collector.

So what can 6MJ (1.8k kWhr) electricity run (the total electricity generated by a 2m2 PV in a day) ?

Energy intensive appliances are heaters or AC or clothes dryers. Relatively low power appliances are fans and lights and fridges. Fridges aren't so much power hungry as energy hungry since they are left on 24/7/365.

A typical bar radiator, single room AC or clothes dryer uses about 1-2kW of power. Therefore the daily stored energy from a 2m2 photovoltaic could run these appliances for at the most 2 hours (1kW x 2 hours = 2 kWhr energy).

On the other hand fans, lights and fridges consume less than 60W and could easily
run for one day to several days on this energy.

All the above calculations are based on the yearly average. In winter expect a 50% reduction in energy generating ability. In summer, you can expect 50% more.

Technological advances may allow us to improve the efficiency of solar collectors cheaply and reliably. Although we are near the practical limit for hot water systems it is theoretically not possible to more than double the current performance of photovoltaics. However we are a long way from that eventuality.

* The energy density of other fossil fuels are ever highter. LPG, petrol or diesel varies from 45-55 MJ/kg.

** Aperture area is the actual portion that harnesses the solar energy and is smaller than the measured collector size.

*** Some important rules of thumb to remember:

1 kW = 3.6 MJ (1000W = kW, 1000 kJ = 1 MJ)
Volumetric heat capacity of water = 0.004 MJ per litre per Kelvin
PV photovoltaic efficiency = 20% (theoretical maximum 45%)
Solar HWS efficiency = 50-80% (temperature dependent)
Average solar insolation (Sydney) = 16 MJ/m2 (8 MJ winter 24 MJ summer)

150W photovoltaic panel occupies 1m2 and generates 1kWh/day