[RE-wrenches] PV Assist

Fri Aug 16 16:38:20 PDT 2013

Hi William, 

I have done many systems in identical situations & share your philosophy. Remote communities, work camps, highway maintenance compounds, etc... 

I have used two general philosophies on batteries for these systems. 

1. Cheap batteries, ie L16's. Cycle them hard and realize they will be dead in 18-24 months. Keeping them cool is the biggest challenge with multiple cycles a day. Special attention to air flow in battery box helps. One site was able to config the room so the cold air intake for generator passes through the battery bank. Worked quite well. Also good for billable hrs to be replacing batteries constantly. 

2. Higher quality flat plate 2V cells. Spend more now, shallow cycle them to around 60-70% DOD, have the gens set to kick in based on high continual AC demand (and shut down when that load abates). Generally speaking the threshold for that is 50% of the inverter rating but each site is unique. Some sites I have the gen may kick in up to 6x a day, not full cycles. Some sites with predictable peaks ie camps have timers to run gen from 6-8am then 5-7pm, to coincide with meals. Though I prefer based on AC demand. More flexible to adapt to the conditions ie if the site is shut down. 

For either config, a single point battery watering system is ideal, as uses a lot of water. Can be automated so the filling system pump runs for xx seconds at the start of each absorb cycle. I've had to go through a lot of junk & few dry cells to find one system that does this reliably. 

Pricing wise over 10 years generally works out about the same, but cash flow is easier for cheap batteries, they just know they will have to spend a couple grand a year on batteries. They are saving 10's of thousands in diesel so still works out economically. 

Generally speaking, inverter capacity is 50% of generator. Several 50 kW diesel / 30 kW inverter / 100 kWh battery systems out there. A larger system we put in 16 months ago uses 100 kW diesels, 60 kW inverter, 400 kWh battery storage. 

I've worked with every inverter out there, and the SMA Sunny Island does the best job of properly charging batteries / and keeping generator runtime as low as possible. It can be set to short-charge batteries most of the time, ie shortened absorb stage and then every x cycles (defined) it will do a full charge. Find that is the very best balance between fuel use / battery life. 

Solar PV (or other renewables) are easy add ons. Every kWh from renewables is saving diesel. 

Hope that is useful. 

Kevin Pegg
EA Energy Alternatives Ltd. 
British Columbia, Canada. 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org]On Behalf Of William Miller
Sent: August 16, 2013 12:49 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] PV Assist

Jay:

There are four systems in question.  In three the loads are year round and one is summer only AC loads.  The year round loaded systems have loads day and night.

One is a highway maintenence station with at least 4 residences.
One is a family compound with two homes with electric ranges, water heaters, AC units (I know its stupid, lord knows I've tried to convince them to change appliances.)
One is an animal rescue operation with heater loads.
One is a large home with 5 very small, very new, very sophisticated and frugal AC units

I know it is silly to move these loads off-grid, but people do it and they will hire someone to work on the systems.  It may as well be me, doing the best I can to use the resources as wisely as possible.

William

PS:  It always happens like this:  I get a call from a client with problems.  Well, of course their expensive battery bank died and it died because of wasteful appliances.  The prescription is a new, expensive battery bank and, at the same time, new, expensive, efficient appliances.  They complain can't afford both and they have to have the batteries, so that is what they get.  Repeat every two years.  Sad....

Wm

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jay Peltz
Sent: Friday, August 16, 2013 12:22 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] PV Assist

Hi William

I feel it's impossible to discuss without more specifics. 

For example if the extreme loads are only every ( just run the genny) so often vs seasonal ( maybe AC coupling makes more sense vs cycling a extremely expensive battery 4 x day),  vs year round vs what is do able etc

I would be curious to know what some of the parameters are, as I'm sure others as well

Jay

Peltz power. 

Sent from my iPhone

On Aug 16, 2013, at 10:53 AM, "William Miller" < william at millersolar.com> wrote:

Friends:

We have been receiving a flurry of requests for design of off-grid systems where the loads are enormous, relative to normal off-grid requirements.  It is obvious that the size of PV and battery arrays to power these loads is beyond what is practical.  We have dubbed these systems Generator/battery with PV assist, or PV assist for short.

These systems will cycle batteries multiple times per day.  It is my understanding that the extra battery cycels will shorten battery life.  Since generator run is expected, we are willing to increase generator run time in order to prolong battery life.

The parameters I suggest are an aggressive load start and a very high battery start parameter.  This will prevent deep discharge of the battery bank.  I know shallow discharge is not the norm, but I don't believe shallow cycling is a problem as long is it as at the top of the voltage window, i.e. the batteries achieve absorption at least once per day.

We know a load start ciuld result in an abbreviated run time, curtailing absorption period, but we are assuming there will be a battery votage start at least once per day allowing full absorption.

Has anyone else considered these issues, and what conclusions did you make

I am looking forward to a spirited discussion as usual.  I throuroughlyenjoy and beneift from them, even thought the advice is usually conflilcting.  Thanks in advance.

William Miller

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