Solar Load shedding/load managment of excess production

[crystallattice]

Hi Guys,

I've seen this question asked before but none with a very conclusive answer so I'll try and make my objectives clear and hopefully get some answers.

I want to install an Off grid PV array with a Morningstar MMPT 60A and a small battery bank. The idea is to charge the batteries first thing in the morning, then after they are full run a AC inverter and run loads like the chest freezer/ fridge, washing machine directly from the array (through the MPPT of course) without discharging the battery. If the loads are done then switch on the water heater and store more energy there. At night the battery will be used for LED lights and small loads.

The purpose of this is to rather invest money in PV than batteries and manage the loads better. I've looked into battery voltage monitoring but this is very crude and not ideal. After reading the MorningStar Tristar 60A MPPT manual it seems the answer might lie in the communications with the controller. The webpage interface shows a "Battery power" and a "Sweep Pmax" values. From a youtube vid (https://www.youtube.com/watch?v=X2XBguHAK_w) at around 1:06 I saw some values of 131W for battery power and 172W for Sweep Pmax. So an excess of 41W, so if I were to control my loads to use say 40W I can increase/decrease depending on the excess. The 1W is left as an indication that the battery gets its fair share, if it becomes zero the load is throttled back and vice versa. I'm thinking X10 control for the loads and phase control for the water heater. Micro-controller does the calculation/ Load priority matrix management.

I've asked the Morningstar tech help if this will work as there are also some other/similar MODBUS variables that can be monitored but I'm still awaiting a reply. The beauty of this is that there is no need for current shunts and it can be done over the network some distance form the MPPT and could also be implemented on other brands of controllers featuring comms and readable variables.

Has anyone looked into using these variables like this? Has anyone gone the current shunt route and done load management doing this. Is this even possible with a MPPT? The only other method I can think of is to monitor current into battery and total current drawn from MPPT but this leaves the question: how much power is really available and how much does the battery want?

Your comments and thoughts are welcome!

Thanks in advance

[YCsolar]

hi, Sir, I guess it is a manual control system, you switch power distribute by yourself, right?
If so, it is simple, don't need monitor current, there will be indication on battery full charge or over discharge.

If you need a automatic control system, hybrid inverter is suitable for you, it will control charging battery, operating loads intelligently and automaticly.
What is capacity of solar power system? What is electricity consumption at night?

[crystallattice]

Not manual, must be automatic and off grid so Hybrid is not an option. Array will be around 1KW, consumption at night will be a few hundred watts at most; TV, DVD, LED lights, cell phone charger, PC

[Mike90250]

The Midnight controller has a "Waste Not" feature, where it can activate a relay when the batteries are in float, if voltage drops too low, it cuts off. Check it out. I've not used that feature

[crystallattice]

Thanks Mike I'll look into that. Problem is still that I can't manage the load, it's either all or nothing with a controller activated relay. Either way I'll look into it.

[SunEagle]

Not manual, must be automatic and off grid so Hybrid is not an option. Array will be around 1KW, consumption at night will be a few hundred watts at most; TV, DVD, LED lights, cell phone charger, PC

Depending on where you live a 1kw array will probably just be able to recharge your batteries from the overnight usage.

If you want to be able to run large loads like the washer, refrigerator, AC, or heat water you need to look into a system greater than 7kw.

[YCsolar]

Depending on where you live a 1kw array will probably just be able to recharge your batteries from the overnight usage.

If you want to be able to run large loads like the washer, refrigerator, AC, or heat water you need to look into a system greater than 7kw.

1KW solar system daily capacity is about 3-5KWH, I don't think have to 7KW system, 2-3KW is enough.

[YCsolar]

Not manual, must be automatic and off grid so Hybrid is not an option. Array will be around 1KW, consumption at night will be a few hundred watts at most; TV, DVD, LED lights, cell phone charger, PC

We can design system for you, if you need, add my skype: <deleted>

--Mod Note: Advertising is not allowed except by contract with forum sponsors. Contact user Jason or Solar Pete.

[crystallattice]

Here is more info to give you a better picture:

My household consumption for a month is about 350KWh. Say about 12KWh per day. My absorption freezer consumes 4KWh/d. Needles to say I will get rid of that so that leaves me with around 8KWh/d. I'm installing solar water heating which could save up to 50% but lets rather work on around 30%. So 8KWh - 30% = 5.6KWh/d. We get an average of 7h sunlight each day. With a 1Kwp array running at say 50% output I'll get 3.5KWh/d. I'm planning on changing over to LED lights running from a different 250Wp circuit which will obviously tip the scales in the positive direction w.r.t. the 1KW production vs consumption.

Changing the efficiency of the solar water heater to 45% and the array output to 65% I'm already producing more than I need (4.4 Cons vs 4.55 production). If I were to use batteries for the 1KW system that would drop my overall efficiency quite a bit so I want to stay away from that as far as possible. My main consumers at night are lights/TV/cellphone chargers - this will hopefully run on the 250W system.

I don't have AC, the freezer will be on timer/load management to run during sunlight hours, washing is done during the day, fridge is a bit of a problem so will have to see how it will pan out.

Electricity costs p.a is around $600. A 1.25KW solar array + mounting is around $1300 + $500 for charge controller + $400 for inverter = $2200 so around 3.6 years to pay back the system. If I were to add batteries, things would change quite a bit. Therefore I want to reduce my reliance on batteries and rather get a few (free) old golf cart batteries and do proper load management. Grid tie is an option to get FIT and reduce storage requirements but again a huge cost (Around $2800) and if the grid is off, which happens a lot, you're stranded.

Your input and comments are welcome.

Edit: I see there were some replies whilst I was typing. Confirms what I just calculated. I would prefer/enjoy to design the system myself but thanks for the offer.

[SunEagle]

1KW solar system daily capacity is about 3-5KWH, I don't think have to 7KW system, 2-3KW is enough.

I would think that any load rated 1 to 2kw (AC, water heater, stove, microwave) could have a considerable starting inrush load. Add any of those two loads together and a 2-3kw array would probably cause the second load to stall out.

Even a small AC unit will draw about 5kw which could not be maintained with only 3kw of array.

The OP would have to be very diligent in making sure what loads can be turned on and what can't throughout the day. That requires a person being very aware, a smart control system measuring loads and available power from the PV or a large PV array to handle multiple loads throughout the day.

Based on the OP's estimated 12kWh /day usage he would probably need closer to a minimum of 5kw array for Winter months and if his battery is too small he would have to go without powering any of the big loads if the sun isn't shining.

[crystallattice]

Well, to elaborate more: I don't have a microwave and don't intend on getting a AC. Only motors that need to start are the fridge/freezer compressors and washing machine motor/pump. The freezer I'm looking to buy is "solar compatible" and only draws 0.3A, the washing machine has a three phase motor for the wash motor and the pump is quite small. If need be I can make some soft starters for the motors and thus eliminate the inrush.

I'll use a micro controller and a priority matrix to do the load management. I've seen a research paper where this has been done. My main problem is determining the amount of power available to be split between the loads. I was hoping someone might shed some light on the Tristar 60 MPPT or other controller's internal variables use in determining the available power.

[donald]

I have a tristar and I don't know how to do what you are suggesting. I also think in your application a precise approach as you are suggesting will not be economical. The guy with the new house who makes a lot of reverse osmosis water would probably benefit economically from the precise approach you are suggesting.

If you don't find a reasonable way to get the excess power measurement, perhaps look at a way to determine if the controller is in absorption or float mode. You could then add power usage based on those states.

A custom refrigerator/freezer could be made to "store cold". But I don't see how you are going to run a store bought unit only on sunny afternoons. If you want to go bleeding edge consider LFP batteries. At least with these there isn't the problem of the desirability of a full charge. Much of the waste you are trying to capture is the result of the characteristics of FLA batteries.

Personally, for a microprocessor controlled off grid home electrical system, I would be looking at relatively large solar, relatively small LFP batteries, and a quality generator. I would design to run the LFP between 20-80/90% DOD. The advantage of LFP is not having to worry about DOD as long as it's in that range.

[crystallattice]

Some valid points there Donald.

How do you find the Tristar and would you do it again? Which model do you have?

It is fairly simple (from the Tristar docs, there are variables you can poll via serial/TCP comms) to determine the state the battery charging cycle is in. I'm trying to determine how much reserve power the PV has for me to utilize without affecting the charge stage. This is not so clear from the docs!

If you do a search for "minus 40" they make freezers that only need about 4 hours of sun per day, no batteries. There are also others. It costs about $2000, an AEG Arctis (A83400HLW0) with a rise time of 58 hours and current draw of 0.3A @ 220V costs about $900. It has massive insulation and would be my fist choice for my needs before I fork out $2000! A similar sized normal chest freezer is about $400 with insulation about 1/3 as thick as the AEG.

When back at the PC I'll try and upload some comparison pics of the side and lid insulation.

LFP is a nice option but so is bigger PV, both cost money though...

You could say if your car is too slow/sluggish you can put a bigger engine in, or you can tune it for better performance!

PS. Electronics is my forte so not much of a hurdle to me as it is to others.

[thastinger]

Do you plan to sit in the dark on a cloudy day? What happens when there are 2 or 3 of them in a row?

You seem to have based your calculations assuming you get full sun for the full hours each day onto your 1Kwh array, just because it is daylight outside doesn't mean your array is pumping out 100% capacity. If you bought 1Kw of panels, you'd be lucky to see 800 out of them most sunny days in the summer time, cloudless winter days will get you closer to rated power but you're never going to run at rated power for very long.

There are no shortcuts to designing an off-grid system, if you need 5Kwh a day, get your wallet out because it'll cost 8-10K. Trust me, I have a system larger than what you're trying to build and it would not do what you're wanting.

[crystallattice]

If it's dark I'll just have to get romantic and light some candles....

I did some conservative calcs in post #9 which would make me believe it should work, cutting it close, but should work. If it rains I don't do washing so less load, it's cool so the freezer/fridge does not have to work so hard. Chest freezer can stay off for at least two days so a small gen can be used in a pinch. If need be I can use the grid, I'm not a tree hugger that wants to claim I'm off the grid. I want to be more self sufficient and try and save money/use it wisely along they way.

Also, a guy at work went off the grid for $25K but made no lifestyle changes/efficiency improvements. If he had made the necessary changes before he converted it would have cost him a lot less.

I have no shame in saying that I'm new to solar but just because everybody spends a small fortune on a PV system does not mean it has to be that way.

Ps. Average sun is about 7h, winter is about 6h I think, quite a bit more than in the states/some other parts of the world if I'm not mistaken.