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"Overgeneration" is not a problem with photovoltaic panels. You simply open circuit the panel and let it sit. In my own off grid solar installation I routinely

by jws 11y ago

"Overgeneration" is not a problem with photovoltaic panels. You simply open circuit the panel and let it sit. In my own off grid solar installation I routinely fill my batteries, and after doing any high power tasks, like vacuum the rugs, the panels just go idle. To a grid connected system it means you can't sell your power during the solar peaks, but it also means that you have enough capacity to sell more power during the edges of the solar periods. Certainly any business which invests in long lived capital equipment would like to operate it at 100% capacity all the time, but most industries realize this doesn't happen. Solar will adjust. For off grid people "excess" capacity means getting by better on partly cloudy days. I'll probably double my capacity next spring for just this reason.

toomuchtodo 11y ago
Could you dump excess energy into a hot water heater? Preheat with thermal on the roof and with excess solar PV power, and then the final temperature lift with propane or natural gas?
- ChuckMcM 11y ago
  Yes, you can do that. One of the more interesting strategies if you have the space for it is to pump water with excess energy. Pump water from a lower reservoir to a higher reservoir, you can later recover that energy by draining the water through a micro-hydro system. One challenge though is capacity, you would probably want essentially two olympic sized pools, one above the other as your storage unit. PG&E in California does this one a fairly large scale in the Bay Area. Pumping water into the Helms Creek reservoir (https://en.wikipedia.org/wiki/Helms_Pumped_Storage_Plant https://en.wikipedia.org/wiki/Helms_Pumped_Storage_Plant)
  - AnimalMuppet 11y ago
    And if you can afford having two olympic swimming pools, you can probably also afford the solar installation. ;-)
  - logfromblammo 11y ago
    Pumped storage is not practical for a typical home. A 1000kg mass, raised 1m off the ground, stores about as much potential energy as a single AA (LR6) battery. The only other energy storage technology that is economically competitive with rechargeable batteries [in 2015] is compressed air. A pressure vessel approximately the same mass and volume as a lead-acid battery bank can store about the same amount of energy. It also has a longer useful lifespan and lower maintenance requirements. But the additional equipment required to connect it to your house power system is more complex and expensive. Rather than stacking two giant swimming pools, you would likely be better off burying a giant concrete sphere wrapped in Spectra/Dyneema/UHMW-PE fiber in your yard, and using a thermo-pneumatic engine to fill or drain the energy reservoir.
    - ChuckMcM 11y ago
      Just pulling some actual numbers about pools, 2.5M liters raised 2 meters, is 50M joules or about 13.8kWH. My house consumes about 24kWH a day so that is about half the power I use. Or put a different way, if during the "solar day" (5 hours of peak sunlight) my system generated 24kW, and my house used 2kW/hr nominally then 10 of those kW would go into the house, the other 14kW into pumping water, which would start draining after the Sun went down providing me enough energy until the next solar day started.
      - logfromblammo 11y ago
        Typical U.S. household electricity use peaks in midsummer and midwinter. Solar generation is higher during the summer, and lower during the winter. So there are several levels of storage: - 100% off-grid will need several days of storage. - Grid-independent will need enough storage to cover the *worst* winter day without resorting to the grid. - Summer-independent will need enough storage to cover the *average* summer day without tapping the grid. - Peak-independent will need enough storage to avoid tapping the grid during daily usage peak times. - Emergency backup will need enough storage to run only critical systems during a grid outage. Home usage usually has a morning ramp-up around 7 AM, a daytime plateau, and a peak around 7 PM, followed by a slow drop-off at night. Homes entirely vacant during the day could probably back off from the plateau with a smart thermostat. You probably want to set up your solar such that it is capable of meeting 100% of your needs on a sunny winter day, and configure storage such that it has capacity equal to three days of typical usage. So for your example, a 5 kW solar array, and 75 kWh (270 MJ storage). So two reservoirs of 2700 m^3 with a 10 m head would do it. To make calculations easier, let's make each reservoir 10 m high. A cylindrical reservoir with diameter 20m, height 20m, and an internal floor 10m high would be about the right size. Or you could use 8 Tesla 10 kWh wall units, costing $28000, and taking up just 1.4 m^3 of space.
  - jws 11y ago
    You've been reading my TODO list! One of my spring projects is to trip the well pump early and pump up the pressurized accumulator tank early when I have surplus power. Ordinarily I let it cycle through a wide range since a gallon stored at low pressure takes me much less energy than a gallon stored against high pressure. More than linearly since it is a centrifugal pump and between the head and the lift I'm pushing the limits. I can't do hot water. I use an on-demand propane unit that can't throttle down enough to accept preheated water. Once that wears out I'll replace it with a unit that can accept preheated water. Two olympic sized pools would be a little decadent plus, I've only got about 12 feet of altitude variation to work with. Constraint of a small island, on the other hand, I do have over 1000 cubic miles of water in the lower pool.
    - toomuchtodo 11y ago
      You live on an island!?! That's AWESOME.

by jws 11y ago

toomuchtodo 11y ago

Could you dump excess energy into a hot water heater? Preheat with thermal on the roof and with excess solar PV power, and then the final temperature lift with propane or natural gas?

ChuckMcM 11y ago
Yes, you can do that. One of the more interesting strategies if you have the space for it is to pump water with excess energy. Pump water from a lower reservoir to a higher reservoir, you can later recover that energy by draining the water through a micro-hydro system. One challenge though is capacity, you would probably want essentially two olympic sized pools, one above the other as your storage unit. PG&E in California does this one a fairly large scale in the Bay Area. Pumping water into the Helms Creek reservoir (https://en.wikipedia.org/wiki/Helms_Pumped_Storage_Plant https://en.wikipedia.org/wiki/Helms_Pumped_Storage_Plant)
- AnimalMuppet 11y ago
  And if you can afford having two olympic swimming pools, you can probably also afford the solar installation. ;-)
- logfromblammo 11y ago
  Pumped storage is not practical for a typical home. A 1000kg mass, raised 1m off the ground, stores about as much potential energy as a single AA (LR6) battery. The only other energy storage technology that is economically competitive with rechargeable batteries [in 2015] is compressed air. A pressure vessel approximately the same mass and volume as a lead-acid battery bank can store about the same amount of energy. It also has a longer useful lifespan and lower maintenance requirements. But the additional equipment required to connect it to your house power system is more complex and expensive. Rather than stacking two giant swimming pools, you would likely be better off burying a giant concrete sphere wrapped in Spectra/Dyneema/UHMW-PE fiber in your yard, and using a thermo-pneumatic engine to fill or drain the energy reservoir.
  - ChuckMcM 11y ago
    Just pulling some actual numbers about pools, 2.5M liters raised 2 meters, is 50M joules or about 13.8kWH. My house consumes about 24kWH a day so that is about half the power I use. Or put a different way, if during the "solar day" (5 hours of peak sunlight) my system generated 24kW, and my house used 2kW/hr nominally then 10 of those kW would go into the house, the other 14kW into pumping water, which would start draining after the Sun went down providing me enough energy until the next solar day started.
    - logfromblammo 11y ago
      Typical U.S. household electricity use peaks in midsummer and midwinter. Solar generation is higher during the summer, and lower during the winter. So there are several levels of storage: - 100% off-grid will need several days of storage. - Grid-independent will need enough storage to cover the *worst* winter day without resorting to the grid. - Summer-independent will need enough storage to cover the *average* summer day without tapping the grid. - Peak-independent will need enough storage to avoid tapping the grid during daily usage peak times. - Emergency backup will need enough storage to run only critical systems during a grid outage. Home usage usually has a morning ramp-up around 7 AM, a daytime plateau, and a peak around 7 PM, followed by a slow drop-off at night. Homes entirely vacant during the day could probably back off from the plateau with a smart thermostat. You probably want to set up your solar such that it is capable of meeting 100% of your needs on a sunny winter day, and configure storage such that it has capacity equal to three days of typical usage. So for your example, a 5 kW solar array, and 75 kWh (270 MJ storage). So two reservoirs of 2700 m^3 with a 10 m head would do it. To make calculations easier, let's make each reservoir 10 m high. A cylindrical reservoir with diameter 20m, height 20m, and an internal floor 10m high would be about the right size. Or you could use 8 Tesla 10 kWh wall units, costing $28000, and taking up just 1.4 m^3 of space.
- jws 11y ago
  You've been reading my TODO list! One of my spring projects is to trip the well pump early and pump up the pressurized accumulator tank early when I have surplus power. Ordinarily I let it cycle through a wide range since a gallon stored at low pressure takes me much less energy than a gallon stored against high pressure. More than linearly since it is a centrifugal pump and between the head and the lift I'm pushing the limits. I can't do hot water. I use an on-demand propane unit that can't throttle down enough to accept preheated water. Once that wears out I'll replace it with a unit that can accept preheated water. Two olympic sized pools would be a little decadent plus, I've only got about 12 feet of altitude variation to work with. Constraint of a small island, on the other hand, I do have over 1000 cubic miles of water in the lower pool.
  - toomuchtodo 11y ago
    You live on an island!?! That's AWESOME.