MaxG
Member
We currently use $250 to heat our water with electricity (on switched tariff).
We are planing to go off-grid... putting heaps of PV panels on the roof, there is enough excess energy to heat the hot water, thus reducing the number and complexity of systems.
However, since we need a tank with hot water in it for hot water and hydronic heating, we could use solar and PV collectors to feed this system?
What are your thoughts?
TonyT
Member
In Adelaide in June the average daily solar insolation onto a horizontal plane (i.e. with a tilt angle of zero degrees) is 2.3kWh/sqm.
At different tilt angles, the June daily solar insolation is:
3.3kWh at 20deg, 3.6kWh at 40deg, 3.6kWh at 50deg, 3.6kWh at 60deg, 3.5kWh at 70deg, 3.4kWh at 80deg, 3.3kWh at 90deg.
http://rredc.nrel.gov/solar/calculators/pvwatts/version1/International/inputv1_intl.cgi
The average daily solar insolation for each month is shown in the following Chart:
.
The value 3.6kWh/sqm per day for the June solar insolation is used in the following calculations.
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SOLAR PV:
The purchase price for solar PV panels, without rebates, without installation and ? without an inverter is ? $1.50/Watt.
Say ? $2250 for a 1.5kW system with 8 panels of mono-crystalline PV covering 10sqm.
Per sqm this is 2250/10 is $225/sqm.
The amount of electrical energy that is collected from mono-crystalline panels is ~12% of the solar insolation.
The average June daily electricity collection from one sqm of PV panel in Adelaide is
0.12 x 3.6 is 0.43kWh/day per sqm.
If this solar electricity replaces grid-electricity bought for 30cent/kWh it is worth
0.43 x 30 is 13cent/day per sqm
If the PV system is not grid-connected, batteries with capacity to store one daily cycle of 0.43kWh per (sqm of PV) are needed so that the electrical energy can be used at the times and in the locations that it is needed.
If CALB (previously Sky Energy) LiFePO4 batteries are cycled between "Full" at 95% and "Re-fill" at 30% they will last for 3000+ cycles (~8 years at one cycle/day).
http://ev-power.com.au/-Sky-Energy-Batteries-.html
To deliver 0.43kWh/cycle, requires LiFePO4 batteries with a storage capacity of
0.43kWh / (0.95-0.30) is 0.67kWh/(sqm of PV).
At 12Volts this is 670W/12V is 56Amp.hours per (sqm of PV collector)
For Lead Acid batteries, the Chart below shows that using a DOD of 70% the battery life is 780 cycles (about 25% of the 3000+ for LiFePO4).
By choosing Lead-Acid batteries the required storage capacity would exceed 100Ah per (sqm of PV) and the life of the Lead-Acid batteries would be less than half that of LiFePO4 batteries.
Instead of installing an excess capacity of batteries, it is cheaper to use a 2.4kVA (petrol) generator (? $1000) to charge the batteries on each cloudy day or whenever additional electricity is needed.
The cost of LiFePO4 batteries is about $500 per kWh.
At 12V one kWh is 1000Wh/12V is 83Amphour.
In summer the daily return from solar PV is at least double the winter return.
A battery comparison with CALB notated as "SE" is at
http://www.evpower.com.au/IMG/pdf/TSvsSE_comparison.pdf
After investing in a CO2 heat pump (a reverse cycle air-conditioner or a heat-pump hot water system), one kWh of electricity can be used to provide up to 4kWh of solar heat.
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COMPARE SOLAR THERMAL:
A purchase price for solar thermal Evacuated Tube (ET) collector panels, without rebates, without installation and without an insulated tank is $1550 for 30 tubes covering 3.8sqm.
i.e. 1550/3.8 is $408/sqm from
http://www.runonsun.com.au/runonsun/Retrofit_or_complete_split_system.html
Alternatively, for an ET collector area of 3.8sqm with 30 tubes (? different tube quality) and including the frame
the price for supply only is $907 or $238/sqm from
http://www.fxlarkin.com.au/page119775.cfm
In Adelaide, in winter (June) the amount of solar energy collected as heat by ET is ? 70% of the solar insolation.
http://www.energymatters.com.au/renewable-energy/solar-power/solar-hot-water/hills-collector-efficiency.pdf
The typical daily heat collection using ET from one sqm in Adelaide in June is
0.70 x 3.6 is 2.5kWh/day per sqm.
If this heat replaces heat from an electric resistance heater using night-tarrif electricity bought for 14cent/kWh it is worth
2.5 x 14 is 35cent/day per (sqm of ET)
If the heat from the ET collector replaces heat from a CO2 heat pump with COP of 4.0,
http://www.ata.org.au/forums/topic/4384
then the heat is worth 14/4 is 3.5cent/kWh so
2.5 x 3.5 is 9cent/day per (sqm of ET)
An insulated tank is needed to store the solar heat energy so that it can be used when and where it is needed.
One kWh of heat is needed to heat 20litres of water by 40degC, e.g. from 20 to 60degC.
If the water is heated by 40degC, the volume of water that is needed to store 2.5kWh of heat is 2.5 x 20Litres is 50Litres of water.
This suggests needing an insulated tank volume of 50Litres per sqm of ET collector, or 200Litres for a typical 30tube ET system that covers ~4sqm.
The cost for a Chinese insulated stainless steel tank is $2.50 to $3.00 per litre.
An Everlast (made in Victoria) ? 327 litre insulated stainless steel tank including a stainless-steel heat-exchanger tube costs ? $1000.
In summer the additional heat that can be captured by solar thermal typically is wasted unless it can be used to heat a swimming pool.
Other links:
http://www.ata.org.au/forums/topic/3253#post-25940
http://www.ata.org.au/forums/topic/3253#post-26134
http://www.ata.org.au/forums/topic/3253#post-26175
arttt
Member
TonyT said:
30 tubes covering 3.8sqm............70% of the solar insolation
Tony i think those 2 numbers dont quite work together. from the link you gave 70% is for the aperture area but 3.8sqm looks more like a gross area.
definition and calculation of aperture area http://www.apricus.com/html/solar_glossary.htm#.Ubgc-vlFaVg
Model specs including dimensions and aperture area.
http://www.apricus.com/html/solar_collector.htm#.Ub5GSPlFaVg
Graphs here show that your 70% is optimistic
http://www.apricus.com.au/file_download/32/Apricus-Technical-Manual.pdf
more info on evac tube efficiency.
http://www.apricus.com/html/solar_collector_efficiency.htm#.Ub5JQ_lFaVg
in that link it talks about IAM (incidence angle modifier) and the graph shows it going above 1 for evac tubes. This must be based on a gross area calculation for that to work.
I think for your simple calcs an area of 2.8sqm and maybe a slightly lower efficiency seems more reasonable.
im not linked to apricus in anyway just found their website informative.
MaxG
Member
Tony, thank you for all the data and your post -- much appreciated ... it shall give me a head start... what puzzled me is your reference to m2. I would have thought that my base unit is kW or kWh, but not m2. I am not constricted by space; I will find the space. The concern is too many systems, while energy can easily be converted.
I will start and post what I come up with.
TonyT
Member
MaxG
At many non-rural locations the limiting factor is the area that is exposed to direct sunlight, without any shadows.
The energy available from the sun comes "per sqm".
All numbers of kW or of kWh result from capturing some of that energy.
A sqm is easier to conceptualize than is a kW or a kWh.
The cost of any collector is proportional to its sqm.
System design calculations will start with the kW and the kWh that you need and work back to the required sqm of collectors.
TonyT
Member
artt
Thanks for your correction: the "3.8sqm" in my post is the Gross Area of ET.
Q1. Staying with Gross Area, does 40% seem reasonable for ET in Adelaide in winter ?
and using Gross Area, does 17% (i.e. 17 x 2.32 is 40) seem reasonable for a Flat Plate collector with selective coating in Adelaide in winter ?
I think that the correction from 70% to 40% makes it clear that Solar PV plus subsequent electric heating of water gives a better return on investment than does solar thermal.
To provide very small daily volumes of hot water (e.g. up to ? 30Litres, enough for one frugal person), consider instant electric resistance heating.
To provide larger daily volumes of hot water (up to ? Litres/day), use a tank fitted with an electric resistance heating element.
To provide very large daily volumes of hot water (or to store many kWh of heat), use a large tank and a CO2 heat pump (costing ? $3,000 for a unit with output 4kW or 80kWh/day if running for 20hrs/day).
The heat ENERGY needed to heat one sqm of floor area for 24hrs ranges between 0.1kWh/day (? 8 star) and 1.0kWh/day (? 2 star).
The heat input POWER needed to heat one sqm of floor ranges between
4W/sqm ((100Wsqm)/24hrs is 4W/sqm) and 40W/sqm (1000Wsqm)/24h).
By using a heat pump operating at a COP of 4.0, the ELECTRICITY input power needed to heat one sqm of floor ranges between 4W/4.0 is 1W/sqm and 40W/4.0 is 10W/sqm.
A heat pump delivering 4kW of heat can provide 40W/sqm to 4,000W/(40W/sqm) is 100sqm.
A 327 litre tank stores and releases 16kWh of heat when the temperature of the water changes between 20 and 60degC.
The brilliant principles that are being used at Mt Best can be used to maximize the COP of a heat pump:
- source the heat at the highest possible temperature (e.g. solar heat previously accumulated in the water in a buried tank at ? 15degC) and
- output the heat at the lowest possible temperature by using hydronic pipes to make the floor into a radiator with a very large area and hence a low temperature (? 30degC).
arttt
Member
Tony,
Im no expert but question 1 seems like a reasonable interpretation of the links in my previous post.
Question 2 i dont quite understand where your number come from. The graph you link to show that a flat plate collector would struggle to get water to 75 degrees in winter in Adelaide (they do quite well in any other parts of the county). Gross area and aperture area are basically the same thing for a flat plate so no adjustment needed. The IAM comes into play. I think those graphs include an IAM adjustment.
Your recommendations for water heating options seems right except the very small usage you probably still want storage so you can make use of your solar or off peak electricity depending on your FiT scheme. Instantaneous I think only makes sense if your usage is very irregular (weekender maybe)
TonyT
Member
MaxG
My engineering mind (linear, single track) is both curse and blessing. 
It has me hooked on Pollack's new insights into water.
This includes his concerns about the accepted definitions for "heat" and for "temperature" (Chapter 10):
http://faculty.washington.edu/ghp/new-book/
TonyT
Member
continuing
http://www.ata.org.au/forums/topic/9614#post-44112
For lead-acid batteries the Chart indicates that to achieve a life of 3300 cycles the depth of discharge is 20%, i.e. the "Refill" point is at 80%.
With a typical battery charger, a lead acid battery is "Full" at ? 90%.
An expensive battery charger is needed to fill a battery to 100%.
Per sqm of PV collector, if using lead-acid batteries to achieve a life of 3300 cycles (~ 10 years) with each cycle delivering 0.43kWh requires a storage capacity of
0.43kWh / (0.90-0.80) is 4.3kWh.
At 12Volts this is 4300W/12V is 358Amp.hours per sqm of PV collector.
(At 48Volts this is 4300/48 is 90Amp.hours per sqm of PV collector)
At 12 Volts this is 358/56 is 6 times the 56Amp.hours of storage (per sqm of PV) required for 3000+ cycles if using 12V LiFePO4 batteries.
If a 100Amp.hour deep-cycle lead acid battery costs $200 then the cost for 350Ah is $700.
This cost for lead-acid batteries is double the $350 cost for 56Ah of LiFePO4 batteries.
For a LiFePO4 (LFP) battery the recommended design-current (for both input and output) is 0.8C:
Quoting from
http://ev-power.com.au/IMG/pdf/berore-you-inquire.pdf
" ... For LFP cells the continuous average current draw should not exceed 0.8C.
... The peak current draw should not exceed 3-4C for more than a few seconds.
You can go higher but battery life will be reduced.
This is true for all prismatic LiFePO4 cells even though they may be rated to 10C or more. "
For a 56Amp.hour LiFePO4 battery 0.8C is 0.8 x 56 is 45Amps.
At a current of 0.8C a battery is charged from zero to 100% of its capacity in 1/0.8 is 1.25hrs.
When charging at 0.8C a (noisy) petrol powered generator needs to run for only 1.25hrs to fully charge an empty LiFePO4 battery.
When charging at 0.8C, to input a cycle of 0.65C the generator will run for 0.65C/0.8C is 0.8hr.
If 45Amps are drawn from a 358Amp.hour battery this discharge rate is 45/358 is 0.13C.
At a current of 0.13C a battery will contain zero charge in 1/0.13 (=358/45) is 7.9hrs.
If discharged at 0.05C (in 20hrs) the capacity of a lead-acid battery is 100%
If discharged at 0.15C (in 7.5hrs) the battery capacity is reduced to 86%.
http://www.starkpower.com/highratedischarge.html
When charging at 0.05C a (noisy) petrol powered generator needs to run for 1/0.05 is 20hrs to charge a Lead-Acid battery from zero to 100% charge.
Charging at 0.05C, to input a cycle of 0.1C the generator will run for 0.1C/0.05C is 2hrs.
.
artt
In a temperate climate, annual thermal collector efficiencies of 60% for an Evacuated Tube collector and 46% for a selective flat plate collector located at Dublin, Ireland are reported at
http://arrow.dit.ie/cgi/viewcontent.cgi?article=1023&context=engschcivart
Benny
Member
MaxG - I think you should continue with your analysis in terms of required kWh/kW and total system cost and ignore the references to sqm. As you say there is no restriction on available roof space, the analysis of "per sq m" is irrelevant. Tony provides a good analysis as usual with much useful info' (apart from the reference to Pollack - don't sidetrack the thread Tony (-;) but I don't think te per sq m stuff is relevant.
If system A can provide 10kWh of useful heat at a total system cost of $1000...
and system B provides 5kWh at a total cost of $1000 them chose system A.
It doesn't matter if system A is only 10% efficient and covers 20sqm while system B is 100% efficient and covers only 1sqm IF YOU HAVE NO RESTRICTION ON SPACE.
Thats why the debate about ET efficency vs flat plate is pretty irrelevant - what matters to most people is how many litres of hot water will I get per $ spent.
MaxG
Member
I understand people have different constraints... however, I am also a root cause analyst. This mean I rather fix the underlying root cause instead of applying band aids. In my case, I am building. I just changed the roof to a north-facing skillion to accommodate the minimum number of PV panels I envision, rather than having a gable roof there running north-east with its ridge line. I am lucky to build in a rural zone, meaning I can plonk things where I want them.
I also understand the relying on one system is a higher risk in case of system failure. (All eggs in one basket.) In principle I think along Benny's line: 10kW is 10kW; may the more economical solution win.
While we move from A to B and the habits remain the same, the environment and construction will be completely different; brick veneer to 8-star passive house.
I can calculate to the n-th degree precision, or I take a rough approach to the numbers, because I do not feel like over-analysing vendor specifications.
What I figured with the different models I have one so far, they pan out very similar.
The underlying thinking / approach is:
a) what is my energy requirement
b) what types of energy sources can I or do I want to use
c) which is the least complex, yet flexible and economical
For:
a) I will have hydronic heating, hot water and electricity requirements
b) I will have a combustion heater, possibly with wet back; PV panels, maybe solar hot water
c) this, I am working on 
Thanks good people; alwasy love a good discussion
TonyT
Member
continuing
http://www.ata.org.au/forums/topic/9614#post-44165
If the cells that make up a battery are connected in series to make a 48Volt battery then one kWh of energy is stored in 1/4 the number of cells that are needed to store one kWh if the cells are connected to make a 12V battery.
i.e.
to store the same amount of energy (kWh) the cost by using a 48V battery is 1/4 the cost of using a 12V battery.
Hi Tony, It looks as if your 'engineering mind' had lost you.
Each battery has a certain storage capacity which can be measured in kWh. 4 batteries have simply 4 times the kWh of one battery no matter if they are connected in parallel or in series.
TonyT
Member
Greetings s2s
Yes: for me too this was counter-intuitive.
It has me intrigued and as usual, I may be wrong ...
However, the economics of off-grid storage improve dramatically if the following logic is correct:
It may explain the reference to a 48V inverter-charger at
http://www.ata.org.au/forums/topic/2265#post-19856
Please test my logic:
Four cells each 3Volts and 10Ah connected in series makes a 12V 40Ah battery which stores
12V x 40Ah is 480Watt.hours is 0.48kWh of energy.
Four batteries each 12V and 40Ah connected in PARALLEL gives a 12V 160Ah battery which stores
12 x 160 is 1920Wh is 1.92kWh of energy.
Sixteen cells each 3Volts and 10Ah connected in SERIES gives a 48V 160Ah battery which stores
48V x 160 is 7680Wh is 7.68kWh of energy.
My "engineering mind" argues that:
if a battery is analogous to a sealed tank and if voltage is analogous to pressure in the tank, then when a tank is holds 4 times the pressure it stores 4 times the energy so a battery at 4 times the voltage stores 4 times the energy.
ghostgum
Member
TonyT,
Four cells 3V 10Ah connected in series makes a 12V 10AH battery storing 0.12kWh of energy.
You don't increase the current capacity by putting them in series, only by putting them in parallel. You increase the watt-hour capacity by the same amount whether you put them in series or parallel, but you don't get to double dip like your first and third calculations.
MaxG
Member
I didn't do anything... blame physics
http://en.wikipedia.org/wiki/Series_and_parallel_circuits
TonyT
Member
MaxG
I enjoyed 12 hours in the ostrich position: 24 would have been better.
The blame goes to that mis-leading unit the "Ampere.hour" which is not independent of the voltage.
Battery capacity expressed in Watt.hours would be less misleading.
TonyT
Member
Update on the Solar Thermal information from
http://www.ata.org.au/forums/topic/9614#post-44112
A low pressure system with 60 horizontal 1.8m long Evacuated Tubes covering a gross area of 7.6sqm is available for $1,100 or $150/sqm.
http://www.ata.org.au/forums/topic/9874#post-44542
http://www.ultimatesolar.com.au/ebay/60t-b2.pdf
The aperture area for one tube is 1.65m x 47mm is 0.08sqm per tube or 4.8sqm for 60 tubes.
An efficiency of 70% based on the aperture area is 70 x 4.8/7.6 is 48% when based on the ET gross area of 7.6sqm.
At different tilt angles, the June daily solar insolation is:
3.3kWh at 20deg, 3.6kWh at 40deg, 3.6kWh at 50deg, 3.6kWh at 60deg, 3.5kWh at 70deg, 3.4kWh at 80deg, 3.3kWh at 90deg.
The value 3.6kWh/sqm per day for the June solar insolation is used in the following calculations.
http://rredc.nrel.gov/solar/calculators/pvwatts/version1/International/inputv1_intl.cgi
At 48% efficiency, the typical daily heat collection from one sqm using ET in Adelaide in June is
0.48 x 3.6 is 1.7kWh/day per sqm.
The heat captured daily by 60 tubes covering 7.6sqm will be 7.6sqm x 1.7kWh/sqm is 13kWh/day.
If this heat replaces heat from an electric resistance heater using night-tarrif electricity bought for 14cent/kWh it is worth
1.7 x 14 is 24cent/day per (sqm of ET)
If house-heating is needed for 200days/year the solar heat is worth 200days x $0.24/day is $48/sqm per year.
The ET cost of $150/sqm is paid back in $150/$48 is 3years.
If the heat from the ET collector replaces heat from a CO2 heat pump with COP of 4.0,
http://www.ata.org.au/forums/topic/4384
then the heat is worth 14/4 is 3.5cent/kWh so
1.7 x 3.5 is 6cent/day per (sqm of ET).
The ET cost of $150/sqm is paid back in $150/$12 is 12years.
