What do you do with excess energy once batteries are charged?

cmorgan · September 2019

Hello.

I'm doing some market research and was wondering what normally happens when your storage batteries are charged but your solar PV is generating? Is that a common thing to have more generation than storage to ensure sufficient power during cloudy days etc? Do the inverters just avoid generating power at that point?

I have a grid-tied system now, without batteries, and often export energy to the grid because I'm not able to consume it all.

I'm wondering if that power is simply not made use of or if there are other systems or approaches beyond batteries that are used or that might be interesting to people with off-grid systems.

Regards,
Chris

Estragon · September 2019

In general, the power is not made beyond what's needed to float batteries and support other load. Off-grid, I try to arrange power-hungry loads like water pumping for such times. Some also use "opportunity" loads such as water heating to make use of the extra pv potential.

petertearai · September 2019

Yep once batterys are full, the excess energy potential just doesn't get used . That's where opportunity loads can be used ie vacume ,washing even having an electric oven to bake scones in the middle of the day .opportunity electric water heating etc etc . An off grid system can be quite wasteful of energy on lots of days ..... but on some days you use everything it can produce , and still not get a battery to 100%.

BB. · September 2019

Welcome to the forum CM,

Yes, normally, one sizes the off grid power system thusly:

First size the battery bank to support your daily average load (typically 4x your daily load--2 days of storage and 50% maximum discharge)
Size the charging sources to keep the battery bank happy--5-10-13-20% of bank AH capacity (100AH bank, 5-13-20 amps of charging current). 5% can work for summer/weekend/emergency power. 10%+ usually required for full time off grid charging.
Size the charging source to recharge the daily usage the next day. Typically based on winter hours of sun (larger array, generally less genset runtime/fuel needed in winter). For daily loads (refrigerator, lighting, etc.) size for 50-65% of base load--Run optional loads (washing machine, irrigation pumping, cooking) when the sun is available.
Yes, for a full time off grid power system, the typical owner will have 2-4x the summer generation that they need, and will scrape by in winter and/or need a backup genset).
Solar is all about location... Solar in the American southwest--Generally lots of sun all year round (except for stormy weather). Live in the great white north, bottom of valleys, in forests)--Solar panels are going to be problematic (during winter--Or if not full sun most of the year, shadows "kill" solar power system output).

Battery banks are expensive and solar panels are cheap (20+ years ago, solar panels were expensive and batteries were relatively cheap). Keeping the battery bank "happy" is the owner's responsibility (load and charging source management). Having extra solar panels (more than your "need") is usually a good thing--Keeps your expensive battery bank from an early death (or being "murdered" by over discharge/under charging).

Some folks do have automated methods of using the "extra solar energy"... One is setting up an electric water heater to preheat hot water with the "free solar energy". Another option is to pump water to a cistern/pond. Figuring out how to use the "excess" energy in a useful manner is not easy. Heating and water pumping can be done with variable energy/RPM pumping--Variable energy to other loads is not usually useful (variable power to TV, Radio, Computer, etc... Browning out a refrigerator compressor will ruin the compressor, etc.).

Storing excess electrical energy for more than 1-3 days (typical off grid battery bank sizing), is usually not economically viable). Similar issues that Utilities have (battery vs pump water storage and dams).

-Bill

MrM1 · September 2019

Heat Water

icarus · September 2019

Time shifting loads is essential. For example, I try not to pump water until the sun is out, leaving it in the evening with a bit low pressure. I also take the opportunity to charge all the other batteries, like phones, laptops/tablets, tools etc. I was also using the 120 vac heating element on my propane fridge when he battery was full, but at 300 watts, it made no sense for the few minutes or hours a day to do the change over so I stopped.

Tony

Photowhit · September 2019

cmorgan said:
I'm doing some market research and was wondering what normally happens when your storage batteries are charged but your solar PV is generating?

Who's market? Are you trying to develop a product?
If I know I'm going to run a surplus I run a water heater. I generate 75-80% of my hot water with excess energy from my array.

cmorgan said:
Is that a common thing to have more generation than storage to ensure sufficient power during cloudy days etc?

With off grid system, yes, they MUST generate more power than they use. If compring an off grid system to a grid tied system, you need roughly 3x the size array to off set the same energy used. The system MUST reach fully charged regularly to protect the life of the battery.

cmorgan said:

Do the inverters just avoid generating power at that point?

Off Grid systems are differn't from a grid tied system. The charging of the battery bank is controller by a charge controller. The inverter usually stays on at all times to run loads, but only as it is called upon to do so... So a system may have a 6000 watt array and a 4000 watt inverter. The array will charge the battery bank through the charge controller, While the inverter may run very small loads most of the time.

cmorgan said:
I have a grid-tied system now, without batteries, and often export energy to the grid because I'm not able to consume it all.

Actually, the grid acts as your battery bank, with very close to 100% of your energy produced either used by you or stored via net metering by the grid for you to use at a lter time.

cmorgan said:
I'm wondering if that power is simply not made use of or if there are other systems or approaches beyond batteries that are used or that might be interesting to people with off-grid systems.

Many of the better charge controllers can start loads at some designated point you set in the charging cycle. So some of us will start a water heater and 'store the energy' as hot water. Or run electric heaters to reduce the energy used by gas heaters and other such loads...

littleharbor2 · September 2019

Vacuuming and doing laundry are perfect opportunity loads. If sized right, by early afternoon your battery should be drawing very little from the PV array but there's lots of potential power available.

mcgivor · September 2019

During infancy stages with marginal storage capacity, opportunity loads were a priority, as storage capacity increased they became less of an issue. Juggling loads may seem useful at first but it soon wears out its welcome, especially if others are involved who have little concern about where the energy comes from.

Just because one has a 100A grid service the inclination is not to max out the service just because it's available, having a system that exceeds needs is just a cushion to relax on, not worrying about potential losses, the potential losses will however help when sun is scarce as long as the battery is capable of accepting the additional capacity.

cmorgan · September 2019

Photowhit said:

cmorgan said:
I'm doing some market research and was wondering what normally happens when your storage batteries are charged but your solar PV is generating?
Who's market? Are you trying to develop a product?
If I know I'm going to run a surplus I run a water heater. I generate 75-80% of my hot water with excess energy from my array

I am trying to develop a product. Residential grid-tied is where I've targeted initially. I'm not sure I can post much information about the system here given forum rules but if I am I'll post more info here like links to the website etc.

I developed a system for my own use to do water heating diversion with excess energy from my grid-tied system. I live in a town with a lower feed-in rate ($0.05/kWh) vs. consumption rate ($0.15/kWh) and both for my own situation and others that are in similar energy markets it seemed like a good way to improve solar ROI and make it financially viable for more people to install solar.

The feedback here on off-grid use has been very helpful!

I'm looking to finish productizing it and to get it onto the market. It's a small company, myself and some occasional contractors I've hired to help with various things, and I'm looking to grow by finding co-founders, funding, potentially interested engineers etc.

The extra energy storage capacity, something like 5kWh of daily energy stored as hot water from the two pilot sites I have, seemed like it could be valuable for off-grid users given the cost of batteries and the typical need for daily hot water usage. Also, if people are using fossil fuels for heating water this could help them move away from that to a purely solar solution.

I'm trying to find out how it might be used in an off-grid situation, the level of interest, what systems it would have to integrate with and the hardware and software changes required to work optimally etc. It does sound like automating the use of excess energy could be a benefit based on the feedback here. I'd appreciate any help or guidance anyone has to offer. It's challenging to build a product like this and I'd love to help make a difference in solar pv adoption.

Regards,
Chris

BB. · September 2019

Chris,

You are welcome to post information about your product and talk about the issues.

A common answer is to run resistive heating when you have "free energy" to dump. However, a heat pump system can be more interesting.

More or less, an air to air heat pump (for A/C and Heating) can get a COP down to below freezing of 4:1 (one unit of heat in, 4 units of heat out)... And if you can store that heat (hot water was popular with heat pumps) or even store the "cold" (freezing water with excess/cheap energy and use the ice for cooling a building during expensive power).

The heat pump water heater--Which has a COP of around 3x seemed like a no brainer product--And many companies did make heat pump based electric water heaters (with normal resistive heating for backup/faster heating when needed)--However, there are several issues. The first is cold air is the "waste product". If you need AC or dehumidification, that can be a nice thing. However, if it gets too cold (under roughly 55F for a "typical" HP water heater, the COP does fall). And some folks have had to vent the HP to the outside to avoid over cooling their home/basement.

The other issue seemed to be that finding HVAC service people that understood a HP water heater seemed to be an issue (or simply too expensive?). And this seemingly wonderful idea has become much less prevalent from manufacturers.

From reading this article, there were a bunch of issues/with GE Geospring HP Water heaters. Some self caused, others the complexity of the system and issues that can spring up in different settings (scale, etc.).:

https://www.treehugger.com/sustainable-product-design/ge-pulls-plug-geospring-heat-pump-hot-water-heater.html

You can also run into issues with GT Solar and battery backup... Some utilities (in the USA and Europe) were concerned that somebody could "buy cheap energy at night", charge their battery bank, and feedback power into the grid during the expensive afternoon/evening time frames. And they made it "illegal" (or at least dragged their feet during permitting) when Hybrid Solar systems (GT+Battery capable Hybrid AC inverters) were planned/installed.

While existing battery technology does not make for a cost effective buy off peak/sell on peak at this time (my math from a few years ago was buying at $0.05 or even $0.00 per kWH, a person had to sell energy at $0.40 or more per kWH to "break even"). Batteries are expensive and have both aging and cycle life issues. Better batteries (and chemistry) can last longer, but they cost more up front--2x more expensive battery lasting 2x longer still costs about the same to operate.

-Bill

icarus · September 2019

I am considering buying a small 120 vac water heater tank to use for preheat on my demand propane. A few 100wh would make a bit of a difference. In my case I would probably manage it manually. Ideally I could set it up with two elements (or two tanks) one with a small wattage draw for times of absorb, and another larger element for times of float.

T

Photowhit · September 2019

cmorgan said:

I developed a system for my own use to do water heating diversion with excess energy from my grid-tied system. I live in a town with a lower feed-in rate ($0.05/kWh) vs. consumption rate ($0.15/kWh) and both for my own situation and others that are in similar energy markets it seemed like a good way to improve solar ROI and make it financially viable for more people to install solar.

I think in the US you are up against 2 problems with any 'new' device.
First most net metering is 1:1, I personally expect that to change over time, but I suspect 80+% perhaps 90+% have net metering at 1:1.

Second any device, wouldn't/couldn't work better than a 'gray box timer' to just run your water heater during the after noon and an hour early in the morning. People willing to add something to their system to save money and run the load primarily during the afternoon already have a device available.

https://youtu.be/DmhiN1-DrSY

Only off grid would more complex device needed. Most people who have a system large enough to make something like water heating viable, would have a charge controller capable of having the needed information and starting a water heater. Indeed, I actually thought someone would have made kits with the proper relay to start various devices, with wiring and the proper connector for various charge controllers. I don't think other outside devices would work better as they would require system information.

cmorgan · September 2019

Photowhit said:

cmorgan said:

I developed a system for my own use to do water heating diversion with excess energy from my grid-tied system. I live in a town with a lower feed-in rate ($0.05/kWh) vs. consumption rate ($0.15/kWh) and both for my own situation and others that are in similar energy markets it seemed like a good way to improve solar ROI and make it financially viable for more people to install solar.

I think in the US you are up against 2 problems with any 'new' device.
First most net metering is 1:1, I personally expect that to change over time, but I suspect 80+% perhaps 90+% have net metering at 1:1.

Second any device, wouldn't/couldn't work better than a 'gray box timer' to just run your water heater during the after noon and an hour early in the morning. People willing to add something to their system to save money and run the load primarily during the afternoon already have a device available.

https://youtu.be/DmhiN1-DrSY

Only off grid would more complex device needed. Most people who have a system large enough to make something like water heating viable, would have a charge controller capable of having the needed information and starting a water heater. Indeed, I actually thought someone would have made kits with the proper relay to start various devices, with wiring and the proper connector for various charge controllers. I don't think other outside devices would work better as they would require system information.

Agreed on the net metering. It depends where you are but I also expect that market to increase over time.

Does that gray box timer help enough? The SolarOp system performs that management at realtime so you can guarantee you won't pull power from the grid. I'm sure the gray box approach helps by shifting into solar generation I'm just wondering by how much.

Regards,
Chris

cmorgan · September 2019

Hi Bill.

I appreciate that. The current site is at https://op.solar.

Btw, I had one of those GE Geospring heat pumps and it was serviced three times in two years. Twice for broken train pans and once for a rusted out condenser. Heatpumps are great though, just not that one :-)

I've also been looking at potential future expansion to control heat pumps and other systems based on surplus generation but don't have anything in place yet around that.

Regards,
Chris

BB. said:

Chris,

You are welcome to post information about your product and talk about the issues.

A common answer is to run resistive heating when you have "free energy" to dump. However, a heat pump system can be more interesting.

More or less, an air to air heat pump (for A/C and Heating) can get a COP down to below freezing of 4:1 (one unit of heat in, 4 units of heat out)... And if you can store that heat (hot water was popular with heat pumps) or even store the "cold" (freezing water with excess/cheap energy and use the ice for cooling a building during expensive power).

The heat pump water heater--Which has a COP of around 3x seemed like a no brainer product--And many companies did make heat pump based electric water heaters (with normal resistive heating for backup/faster heating when needed)--However, there are several issues. The first is cold air is the "waste product". If you need AC or dehumidification, that can be a nice thing. However, if it gets too cold (under roughly 55F for a "typical" HP water heater, the COP does fall). And some folks have had to vent the HP to the outside to avoid over cooling their home/basement.

The other issue seemed to be that finding HVAC service people that understood a HP water heater seemed to be an issue (or simply too expensive?). And this seemingly wonderful idea has become much less prevalent from manufacturers.

From reading this article, there were a bunch of issues/with GE Geospring HP Water heaters. Some self caused, others the complexity of the system and issues that can spring up in different settings (scale, etc.).:

https://www.treehugger.com/sustainable-product-design/ge-pulls-plug-geospring-heat-pump-hot-water-heater.html

You can also run into issues with GT Solar and battery backup... Some utilities (in the USA and Europe) were concerned that somebody could "buy cheap energy at night", charge their battery bank, and feedback power into the grid during the expensive afternoon/evening time frames. And they made it "illegal" (or at least dragged their feet during permitting) when Hybrid Solar systems (GT+Battery capable Hybrid AC inverters) were planned/installed.

While existing battery technology does not make for a cost effective buy off peak/sell on peak at this time (my math from a few years ago was buying at $0.05 or even $0.00 per kWH, a person had to sell energy at $0.40 or more per kWH to "break even"). Batteries are expensive and have both aging and cycle life issues. Better batteries (and chemistry) can last longer, but they cost more up front--2x more expensive battery lasting 2x longer still costs about the same to operate.

-Bill

Photowhit · September 2019

cmorgan said:

Does that gray box timer help enough? The SolarOp system performs that management at realtime so you can guarantee you won't pull power from the grid. I'm sure the gray box approach helps by shifting into solar generation I'm just wondering by how much.

It would be 'mostly effective and preserve having hot water when needed. I doubt seriously if someone would pay (assumedly) more for the chance to NOT have hot water available, which is the only conclusion you could have to something that would run only when solar was available ( "...won't pull power from the grid"). Indeed that is why I added an hour of heating in the morning before available sun for the gray box. because typical uses are at night for washing and showers either at night or in the morning.

mvas · September 2019

In my state, and many other states, the maximum hot water temperature allowed by law is 120 F degrees.
Does your device allow the temperature in the hot water tank to rise to 150 F degrees?
If true then how are you getting around the anti-scalding law?

BB. · September 2019

Anti Scald / Thermal mixing Valve?:

https://www.homedepot.com/p/Honeywell-Thermostatic-Mixing-Valve-AM101C-US-1/202264710

-Bill

mike95490 · September 2019

There are water tempering mixer valves. I have one installed between my solar water heater and the inlet to my tankless. Summer, I often see 140F in the roof tank, and sometimes150F.

https://www.caleffi.com/usa/en-us/catalogue/mixing-valves-5

cmorgan · September 2019

Photowhit said:

cmorgan said:

Does that gray box timer help enough? The SolarOp system performs that management at realtime so you can guarantee you won't pull power from the grid. I'm sure the gray box approach helps by shifting into solar generation I'm just wondering by how much.

It would be 'mostly effective and preserve having hot water when needed. I doubt seriously if someone would pay (assumedly) more for the chance to NOT have hot water available, which is the only conclusion you could have to something that would run only when solar was available ( "...won't pull power from the grid"). Indeed that is why I added an hour of heating in the morning before available sun for the gray box. because typical uses are at night for washing and showers either at night or in the morning.

Totally agree on preserving water when needed, my wife wouldn’t be a fan if she couldn’t take a hot shower in the early morning.

The system uses an external sensor to ensure that a minimum tank temperature is met with plans to enhance that to enable even lower temperatures during particular hours for additional energy storage.

On the issue of output water temperature a thermal mixing valve is used to ensure a safe temperature. Has the advantage of ensuring hot enough tank temperatures to prevent legionella from being able to grow in the tank. A concern in Europe but for whatever reason not in the US.

Regards,
Chris

BB. · September 2019

From the reports I have seen, most Legionella reports with water heaters seemed to be related to electric hot water heaters.

My theory (no facts to backup) is that natural gas/propane water heaters heat up a large surface area of the tank (bottom and center stack/heat exchanger) vs the "isolated" Calrod unit(s) in the middle of the tank. If the Legionella lives in the debris at the bottom of the tank, then the lack of heating (sterilization) at the base of the water heater could be the problem.

Just a quick search shows that other(s) may also have an issue with electric water heaters and the bottom of the tank which never gets "hot" (cold water settling at the base of the tank):

https://amwater.com/corp/legionella-homeowners

Electric hot water heaters (right) have the heating element slightly above the bottom of the heater and because hot water rises, water at the bottom of the tank can be a cooler temperature that is optimum for Legionella growth. Homes with high risk individuals may consider installing tankless water system set such that the temperature at the shower head or tap is 122 deg F, which both reduces the risk of scalding and prevents the growth of Legionella bacteria.

I don't know the mix between electric vs natural gas/propane water heaters in Europe--But another question would be do folks in Europe keep their water heaters cooler because of the high cost of energy (in general)?

-Bill

mvas · September 2019

BB. said:

Anti Scald / Thermal mixing Valve?:

https://www.homedepot.com/p/Honeywell-Thermostatic-Mixing-Valve-AM101C-US-1/202264710

-Bill

Thanks.
My hot water tank is already set at 120 F degrees.
This mixing valve now adds another ~$100.00 to this project, lowering the ROI.
What is the total cost of this project ?

cmorgan · September 2019

mvas said:

BB. said:

Anti Scald / Thermal mixing Valve?:

https://www.homedepot.com/p/Honeywell-Thermostatic-Mixing-Valve-AM101C-US-1/202264710

-Bill

Thanks.
My hot water tank is already set at 120 F degrees.
This mixing valve now adds another ~$100.00 to this project, lowering the ROI.
What is the total cost of this project ?

The fully installed price is estimated around $1200 including plumbing, electrical work, the SolarOp module, energy monitor, and mixer valve. For people in high energy differential areas it’s about three years to break even.

How would you imagine calculating the value for an off grid user? How does that pricing sound?

Regards,
Chris

mvas · September 2019

cmorgan said:

Hi Bill.

I appreciate that. The current site is at https://op.solar.

Btw, I had one of those GE Geospring heat pumps and it was serviced three times in two years. Twice for broken train pans and once for a rusted out condenser. Heatpumps are great though, just not that one :-)

I've also been looking at potential future expansion to control heat pumps and other systems based on surplus generation but don't have anything in place yet around that.

Regards,
Chris

Your website claims ...
your device will save more than $450.00 / year (avg cost to make Hot Water) for homes that already have Solar Panels.

That cannot be true, since the energy to make Hot Water from sunrise to sunset, already comes from those PV Panels.
Adding your device will only save, some of the costs, of operating the Hot Water tank at night, by over-heating the water during the day.
Your device "time-shifts" most of the night time Hot Water energy usage, to the day time.
But if you have Solar Panels and you have a low or no FITS, then you have already time-shifted your Hot Water usage to the daytime.
Adding your device will not save the average Solar equipped home, an extra $450.00 / year.

NANOcontrol · September 2019

I remember some advice, Don't go into a business you don't understand. From the responses most don't have any understanding of heating water. People hold onto the first ideas they understand and hold onto them like Grim Death. I'm off grid and heat water with excess PV and it works great. PV systems waste tons of power. and I can harvest as little as 5W. It all adds up.

Legionaries has been with us since man first walked. It is everywhere and we are exposed to it daily. It is a little bit of truth and a bunch of hokum. No one ever heard of it till management discovered they could make their performance numbers look better if they skipped maintenance. You really have to be bombarded for this bacteria to do any damage. If this was actually a problem it would be a major health crisis. If you are collecting rain water or using well water you are providing a source of bacteria and nutrients. Then you better be careful. City water is treated and is not a health risk. There are plenty of places in your piping system that are the right temperature for growing this, it isn't just the water tank. If the water tank gets above 120F regularly, it is sanitized. In the effort to save energy there are people who turn down the temperature to near 100F. It was uncomfortable to shower at my parents it was so cold. I also wouldn't eat anything out of their fridge. They lived to their mid 90's. Bacteria didn't get them. WHO covers a lot of ground and there are many places this could be a problem. If you have a decent water system it is not a realistic problem. This trend to ultra sanitation has only lead to undeveloped immune systems and increased allergies.

cmorgan · September 2019

mvas said:

cmorgan said:

Hi Bill.

I appreciate that. The current site is at https://op.solar.

Btw, I had one of those GE Geospring heat pumps and it was serviced three times in two years. Twice for broken train pans and once for a rusted out condenser. Heatpumps are great though, just not that one :-)

I've also been looking at potential future expansion to control heat pumps and other systems based on surplus generation but don't have anything in place yet around that.

Regards,
Chris
Your website claims ...
your device will save more than $450.00 / year (avg cost to make Hot Water) for homes that already have Solar Panels.

That cannot be true, since the energy to make Hot Water from sunrise to sunset, already comes from those PV Panels.
Adding your device will only save, some of the costs, of operating the Hot Water tank at night, by over-heating the water during the day.
Your device "time-shifts" most of the night time Hot Water energy usage, to the day time.
But if you have Solar Panels and you have a low or no FITS, then you have already time-shifted your Hot Water usage to the daytime.
Adding your device will not save the average Solar equipped home, an extra $450.00 / year.

Agreed on the FITS conditions and those are noted on the project page, or at least intended to be.

The system is designed for people with an imbalance in consumption vs feed in. There are a number of markets where solar pv companies avoid because the mismatch is enough that they ignore and focus instead on the 1:1 net metering areas. I live in such an area.

If you live where I am, and taking into account cloudy days and other inefficiencies it’s still in the ballback of $450/yr in terms of reduction in consumption due to time shifting and making better use of your solar pv generated energy.

Chris

What do you do with excess energy once batteries are charged?

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