12 ms·

Why does this project need battery storage? The power demand for air-conditioning during the day follows the supply of solar. Is it to cater for hot but cloudy

by geertj 9y ago

Why does this project need battery storage? The power demand for air-conditioning during the day follows the supply of solar. Is it to cater for hot but cloudy days?

lultimouomo 9y ago
You can get very hot and humid days when it's overcast.
- walrus01 9y ago
  Yes, with complete overcast even a very large PV array such as quantity 80, 370W high-efficiency 72-cell panels, will not produce more than 3 to 5kW. In bright sun it'll be well above 23kW. Under STC (standard test conditions) that's a 29.6kWh array... what you should care about is your kWh/month produced.
foota 9y ago
I know electricity is very expensive in Hawaii since it's almost all natural gas. Maybe they charge the batteries off school hours to save on the electricity bill?
- walrus01 9y ago
  For large loads, at $0.32 to $0.50/kWh in Hawaii it is now much more economical to disconnect from the grid entirely, unless you have some limiting factor that would prohibit large scale low cost mounting of a huge number of 1.65 x 1.0 (60-cell) or 1.99 x 1.0 meter (72-cell) size high efficiency PV panels. The ROI can be well under 6 years.
  - foota 9y ago
    Sure, but if you can save up extra electricity from your pv when you don't need to cool the building, it doesn't hurt.
  - pjc50 9y ago
    Does Hawaii allow you to sell electricity back to the grid, or it that prevented by the local electricity monopoly?
  - yazr 9y ago
    What do we mean by "high efficiency" PV panels in the context of commercial roof tops ? AFAIK thin film are around 10%, standard PVs around 20% and the multi junction are 30% but cost x10 for special uses only..
    - walrus01 9y ago
      In this context I am not referring to lower-cost, but lower efficiency thin films, nor triple junction or concentrator type cells... High efficiency = 5W STC rating per 156mm cell or better. More W per square meter. For example you can get cheap 72-cell panels that are rated at 320W, the best ones which are exactly the same dimension will be rated at 370, 375 or 380W. That is for typical modules which measure exactly 1.99 x 0.99 meters. In large-scale solar, but not large enough to use a huge land area and low-cost thin films, there are two standard sizes of panels. 60-cell 1.65 x 0.99m and 72 cell 1.99 x 0.99 meter. These are standardized to work with a wide variety of different mounting systems. There are many different grades and quality levels of polycrystalline and mono crystalline silicon solar panels. The very best monocrystalline Si panels are made by Sunpower, but they have a significant price premium as compared to a pallet load that I could buy right now with my Visa card of qty 20, 370W panels. The commodity 370W are 65% of the price but only a few % less efficient.
    - olegkikin 9y ago
      Your numbers are outdated. Thin film = 23%, multi-junction are 46%. "Standard PV" is probably what's available in bulk and cheap, is around 22-25%. https://upload.wikimedia.org/wikipedia/commons/0/01/PVeff%28rev171030%29.jpg https://upload.wikimedia.org/wikipedia/commons/0/01/PVeff%28...
      - walrus01 9y ago
        Those are best in class research cell efficiency for each technology type. The silicon cell based panels you can buy now are 17.5-22% unless you are spending a lot of money. Special thin films cells in a lab may be 23%, what you can buy economically for a grid scale utility power plant is more like 14-15%. This is at the very top end of the thin film market for what is now commercially available, and is 17%. See datasheet. http://www.firstsolar.com/en/Modules/Series-6 http://www.firstsolar.com/en/Modules/Series-6
- melling 9y ago
  I thought oil was used in Hawaii? https://www.washingtonpost.com/graphics/national/power-plants/ https://www.washingtonpost.com/graphics/national/power-plant...
  - foota 9y ago
    Sorry, you are correct.
- amelius 9y ago
  So where is the cost break even point?
walrus01 9y ago
You can't run air conditioners directly off solar. PV panels natively produce DC. There are units with microinverters built into the back of the panel that produce AC. Even in a sunny climate clouds will pass in front of the sun. Unless you have a massively oversized PV array, you can't just run the DC array's output into a bank of sinewave inverters and then straight into the input of multiple 2 to 5kW load air conditioners. I'm assuming here that a typical AC unit will look something like a 12000, 18000 or 24000 BTU/H LG, Daikin or Samsung split type, ductless, cooling-only unit. The cumulative kWh/day and kWh/month produced by your PV is what you should care about. As compared to what your load will consume in the same time period. For off grid, which now economically makes sense in much of Hawaii because fossil-fuel powered grid power can costs $0.32 to $0.50/kWh. You need a battery buffer so that your load (the air conditioners) doesn't brown out and fail when the a few clouds pass in front of your PV panels. At a given time when it is a little bit cloudy out you might have 12kW of load but even a very big PV array might only be producing 4kW. The load is constantly running off the battery and inverter system, while the PV panels and charge controller are constantly doing their best to keep the battery full.
- bringtheaction 9y ago
  You used the word PV a lot but never said what it means. For anyone wondering, PV means photovoltaic. https://en.wikipedia.org/wiki/Photovoltaics https://en.wikipedia.org/wiki/Photovoltaics
  - walrus01 9y ago
    Yes, I forget that not everyone know the term. Work with solar on a regular basis and the whole industry is just as full of acronyms as x86-64 hardware.
    - wernercd 9y ago
      Any developed industry has it's own terms and language - healthcare, military, government, programming, finance... Always fun learning new languages inside languages.
- 9y ago
ZeroGravitas 9y ago
Tesla's latest thing with powerwalls is to build "virtual power plants". This means the batteries are all under central control and can absorb extra power if wind/solar/nuclear are providing more than needed and later reduce load by running from battery when loads are higher. In this case the schools probably just provide a convenient set of distributed locations, owned by a single authority. The money they earn from providing this service can be partially kicked back to the schools to offset air con costs.
SeanOC 9y ago
a) PR b) From what I understand, Hawaii's power utility either doesn't allow or strongly discourages new net metering customers (e.g. where excess generated power goes back to the grid and you get credited). Accordingly, on-site battery systems make sense for most PV solar installs to help deal with the difference in generation and usage over time.
kurthr 9y ago
Net metering is no longer supported in Hawaii, Several different programs have been implemented that are more profitable for the utility. As a result, without batteries and self-supply it is no longer nearly as cost-effective to install solar even at $0.35-0.50/kWh. The utility will only buy at ~$0.10 and sell at 3-5x that rate. https://www.greentechmedia.com/articles/read/rooftop-solar-in-hawaii-crashes-with-loss-of-net-metering-lack-self-supply https://www.greentechmedia.com/articles/read/rooftop-solar-i... https://www.hawaiianelectric.com/clean-energy-hawaii/producing-clean-energy/customer-renewable-programs/customer-grid-supply-plus https://www.hawaiianelectric.com/clean-energy-hawaii/produci...
bkanber 9y ago
They're not running the air conditioners directly from the solar. They're using solar power to offset the energy cost of the AC.

by geertj 9y ago

Why does this project need battery storage? The power demand for air-conditioning during the day follows the supply of solar. Is it to cater for hot but cloudy days?

lultimouomo 9y ago

You can get very hot and humid days when it's overcast.

walrus01 9y ago
Yes, with complete overcast even a very large PV array such as quantity 80, 370W high-efficiency 72-cell panels, will not produce more than 3 to 5kW. In bright sun it'll be well above 23kW. Under STC (standard test conditions) that's a 29.6kWh array... what you should care about is your kWh/month produced.

foota 9y ago

I know electricity is very expensive in Hawaii since it's almost all natural gas. Maybe they charge the batteries off school hours to save on the electricity bill?

walrus01 9y ago
For large loads, at $0.32 to $0.50/kWh in Hawaii it is now much more economical to disconnect from the grid entirely, unless you have some limiting factor that would prohibit large scale low cost mounting of a huge number of 1.65 x 1.0 (60-cell) or 1.99 x 1.0 meter (72-cell) size high efficiency PV panels. The ROI can be well under 6 years.
- foota 9y ago
  Sure, but if you can save up extra electricity from your pv when you don't need to cool the building, it doesn't hurt.
- pjc50 9y ago
  Does Hawaii allow you to sell electricity back to the grid, or it that prevented by the local electricity monopoly?
- yazr 9y ago
  What do we mean by "high efficiency" PV panels in the context of commercial roof tops ? AFAIK thin film are around 10%, standard PVs around 20% and the multi junction are 30% but cost x10 for special uses only..
  - walrus01 9y ago
    In this context I am not referring to lower-cost, but lower efficiency thin films, nor triple junction or concentrator type cells... High efficiency = 5W STC rating per 156mm cell or better. More W per square meter. For example you can get cheap 72-cell panels that are rated at 320W, the best ones which are exactly the same dimension will be rated at 370, 375 or 380W. That is for typical modules which measure exactly 1.99 x 0.99 meters. In large-scale solar, but not large enough to use a huge land area and low-cost thin films, there are two standard sizes of panels. 60-cell 1.65 x 0.99m and 72 cell 1.99 x 0.99 meter. These are standardized to work with a wide variety of different mounting systems. There are many different grades and quality levels of polycrystalline and mono crystalline silicon solar panels. The very best monocrystalline Si panels are made by Sunpower, but they have a significant price premium as compared to a pallet load that I could buy right now with my Visa card of qty 20, 370W panels. The commodity 370W are 65% of the price but only a few % less efficient.
  - olegkikin 9y ago
    Your numbers are outdated. Thin film = 23%, multi-junction are 46%. "Standard PV" is probably what's available in bulk and cheap, is around 22-25%. https://upload.wikimedia.org/wikipedia/commons/0/01/PVeff%28rev171030%29.jpg https://upload.wikimedia.org/wikipedia/commons/0/01/PVeff%28...
    - walrus01 9y ago
      Those are best in class research cell efficiency for each technology type. The silicon cell based panels you can buy now are 17.5-22% unless you are spending a lot of money. Special thin films cells in a lab may be 23%, what you can buy economically for a grid scale utility power plant is more like 14-15%. This is at the very top end of the thin film market for what is now commercially available, and is 17%. See datasheet. http://www.firstsolar.com/en/Modules/Series-6 http://www.firstsolar.com/en/Modules/Series-6
melling 9y ago
I thought oil was used in Hawaii? https://www.washingtonpost.com/graphics/national/power-plants/ https://www.washingtonpost.com/graphics/national/power-plant...
- foota 9y ago
  Sorry, you are correct.
amelius 9y ago
So where is the cost break even point?

walrus01 9y ago

You can't run air conditioners directly off solar. PV panels natively produce DC. There are units with microinverters built into the back of the panel that produce AC. Even in a sunny climate clouds will pass in front of the sun. Unless you have a massively oversized PV array, you can't just run the DC array's output into a bank of sinewave inverters and then straight into the input of multiple 2 to 5kW load air conditioners. I'm assuming here that a typical AC unit will look something like a 12000, 18000 or 24000 BTU/H LG, Daikin or Samsung split type, ductless, cooling-only unit. The cumulative kWh/day and kWh/month produced by your PV is what you should care about. As compared to what your load will consume in the same time period. For off grid, which now economically makes sense in much of Hawaii because fossil-fuel powered grid power can costs $0.32 to $0.50/kWh. You need a battery buffer so that your load (the air conditioners) doesn't brown out and fail when the a few clouds pass in front of your PV panels. At a given time when it is a little bit cloudy out you might have 12kW of load but even a very big PV array might only be producing 4kW. The load is constantly running off the battery and inverter system, while the PV panels and charge controller are constantly doing their best to keep the battery full.

bringtheaction 9y ago
You used the word PV a lot but never said what it means. For anyone wondering, PV means photovoltaic. https://en.wikipedia.org/wiki/Photovoltaics https://en.wikipedia.org/wiki/Photovoltaics
- walrus01 9y ago
  Yes, I forget that not everyone know the term. Work with solar on a regular basis and the whole industry is just as full of acronyms as x86-64 hardware.
  - wernercd 9y ago
    Any developed industry has it's own terms and language - healthcare, military, government, programming, finance... Always fun learning new languages inside languages.
9y ago

ZeroGravitas 9y ago

Tesla's latest thing with powerwalls is to build "virtual power plants". This means the batteries are all under central control and can absorb extra power if wind/solar/nuclear are providing more than needed and later reduce load by running from battery when loads are higher. In this case the schools probably just provide a convenient set of distributed locations, owned by a single authority. The money they earn from providing this service can be partially kicked back to the schools to offset air con costs.

SeanOC 9y ago

a) PR b) From what I understand, Hawaii's power utility either doesn't allow or strongly discourages new net metering customers (e.g. where excess generated power goes back to the grid and you get credited). Accordingly, on-site battery systems make sense for most PV solar installs to help deal with the difference in generation and usage over time.

kurthr 9y ago

Net metering is no longer supported in Hawaii, Several different programs have been implemented that are more profitable for the utility. As a result, without batteries and self-supply it is no longer nearly as cost-effective to install solar even at $0.35-0.50/kWh. The utility will only buy at ~$0.10 and sell at 3-5x that rate. https://www.greentechmedia.com/articles/read/rooftop-solar-in-hawaii-crashes-with-loss-of-net-metering-lack-self-supply https://www.greentechmedia.com/articles/read/rooftop-solar-i... https://www.hawaiianelectric.com/clean-energy-hawaii/producing-clean-energy/customer-renewable-programs/customer-grid-supply-plus https://www.hawaiianelectric.com/clean-energy-hawaii/produci...

bkanber 9y ago

They're not running the air conditioners directly from the solar. They're using solar power to offset the energy cost of the AC.