Uganda, System review

[Manafwa]

I am assisting, as a volunteer, with a solar feasibility study design and budget pricing for a small hospital in Uganda. Would anyone please review the factors and assumptions below and let me know what I have missed.
The hospital has grid power but is unreliable and would like to totally convert to an off grid solar system. The hospital is in a very rural area. System needs to be robust, parts are difficult to get, basically have to be hand carried from overseas visitors/volunteers.

They require 24kWh per day. I am new to solar but I have watched a few youtube videos and read some of this forum and determined the following:

Batteries:
Operating temp factor 1( location will not be below 80 Deg F)
Autonomy days 1.5
Batter discharge limit 50%
Inverter efficiency 92%
Battery needs to provide 78,250 kWh
Battery voltage 48V
Battery Bank Capacity 1,630.43 Ah
Connection strings max =2
Battery Rating needed 815.22 Ah
Need 48 Batteries (2 parallel * 24 in a series string)
Is this right? price seems daunting. How to lower the cost. What would be a good battery selection?

Panels:
Sun hours at location 5.5, location almost on the equator.( I have not found a confirmed data source for this)
Panel efficiency 67% (dusty area soiling, no shade in area, voltage drop, controller efficiency and battery losses). This sounds like a high factor.
Power needed daily 24kWh
Gross panel Watts required 6,512 W (24,000/5.5 at 67% efficiency)
Choose 260W poly panels, (Astronergy VIOLIN ASM6610P-260(BF) Black Frame Poly Pallet (22) of Solar Panels) (https://www.wholesalesolar.com/18900...f-solar-panels)
Will use MPPT charge controller so need panel Voltage to be > system voltage of 48V. So two panel strings required. Total panels required = 26.
Size and weight is acceptable for existing roof.
What other panel consideration for a rural dusty area.

Charge Controller:
I didn't start on the controller until the panels and batteries are confirmed.

thanks,

Steve

[Mike90250]

A battery for an electric forklift (48V) is made from large 2V cells, and will reduce the number of battery caps to check fluid with.

Largest problem is related to insisting only distilled water be used for refilling cells. Beware the local shaman that gets the plant engineer to use "special juice" in the batteries, or the "add an aspirin" to the cells.

Your PV array size is too small. In summer, I use 11KWh, and have a 5Kw array to feed it, I;d like to see closer to 9Kw of PV, split into 3, 3Kw arrays on 3 60A charge controllers.

Setting the PV panels to be washable (road dust) yet vandal resistant, will be the harder part,

[NEOH]

Manafwa
Are you planning on consuming 24 KWH per day and generating 24 KWH per day?
Now let's say you just had 1.5 days of autonomy, therefore the 50% DOD Battery Bank now needs 39 KWH to get back to 100% SOC.
So where is this extra 39 KWH to recharge the battery bank going to come from?
Of the 24 KWH generated - 12 KWH will be consumed immediately during the 12 hours of daylight and the other 12 KWH will be consumed at night.
The Battery Bank will be down to 50% DOD again the very next morning, forever.
Where is your 1.5 days of autonomy now?
Habitual PSOC will cause early battery death.
Doesn't the PV Array need to generate more than the 24 KWH daily consumption to "make up" for any previous cloudy days / excess battery discharge?

Battery at 100% SOC = 78 KWH
Battery at 50% SOC = 39 KWH
12 KWH extra PV daytime energy put into batteries and then used every night.
51 KWH = ( 39KWH + 12 KWH ) is max you will ever have in battery bank after 1.5 days of autonomy, forever.
The next morning the battery bank will be discharged to 50% DOD = 39 KWH

To get Battery Pack back to 100% SOC the very next day the PV panels must generate an EXTRA 27 KWH = 78 KWH - 51 KWH
This requires a PV array capable of 51 KWH = 27 KWH (battery) + 24 KWH (daily)

To get back to 100% SOC in two days requires and extra 14 KWH of PV energy
This requires a PV array capable of 38 KWH = 14 KWH (battery) + 24 KWH (daily)

To get back to 100% SOC in three days requires and extra 9 KWH of PV energy
This requires a PV array capable of 33 KWH = 9 KWH (battery) + 24 KWH (daily)

The PV Array needs to generate ALL of the daily needs PLUS extra to catch-up / make-up ALL or PART of any excess battery drain from previous days.

How many hours does it take to get the battery bank back to 100% SOC via Bulk & Absorb, after one normal nightly drain of 12 KWH
Can the charge controllers accomplish this between sunrise and sunset on the shortest days?

Also, the battery bank is not 100% efficient.
The efficiency of a FLA can vary, from (approx) 95% during Bulk, down to (approx) 80% during Absorb.
Efficiency decreases with increasing SOC.

[Guest]

Thank you NEOH. I see what say. I will revise my numbers and repost. Many thanks

[NEOH]

PV Watts provides monthly insolation for areas around the world ...

http://pvwatts.nrel.gov/pvwatts.php

Kakamega, Kenya ...

Solar Hours
=========
J = 6.96
F = 6.87
M = 6.06
A = 5.10
M = 4.61
J = 4.41
J = 4.52
A = 5.09
S = 5.87
O = 6.10
N = 6.07
D = 6.85
==========
Avg = 5.71

[Manafwa]

Another general question. I assumed that all power would have to pass through the battery to the inverter for use. It this accurate. If the power generated by the panels was used as generated during the day and the storage capacity was only sufficient to provide night time power, wouldn't this half the battery size? Is this possible?
thanks

[Mike90250]

Another general question. I assumed that all power would have to pass through the battery to the inverter for use. It this accurate. If the power generated by the panels was used as generated during the day and the storage capacity was only sufficient to provide night time power, wouldn't this half the battery size? Is this possible?
thanks

That works until there is a cloudy day, then you run out of power, Best battery sizing is for at least 36 hours before low voltage shuts gear down. Less than that, and you are deeply cycling batteries and shortening their life

[Sunking]

I am assisting, as a volunteer, with a solar feasibility study design and budget pricing for a small hospital in Uganda.

They require 24kWh per day. I am new to solar but I have watched a few youtube videos and read some of this forum and determined the following:

Batteries:
Operating temp factor 1( location will not be below 80 Deg F)
Autonomy days 1.5
Batter discharge limit 50%
Battery needs to provide 78,250 kWh
Battery voltage 48V
Battery Bank Capacity 1,630.43 Ah

None of this makes any sense. If you are using 1.5 days autonomy (36 Kwh) is a plan for a destroyed battery. You do not have enough power for even one day. Minimum Autonomy is 5 days which gives you 3 days of usable capacity for those cloudy spells.

If they require 24 Kwh/day as you state and use a 48 volt battery: [ 5 days x 24,000 watt hours] / 48 volts = 2500 AH @ 48 volts.

Minimum size panel wattage to keep a 48 volt 2500 AH battery is 10,400 watts will deliver 200 amps of charge current @ C/12

A system this size really needs to run at 100+ volts DC battery.

[Manafwa]

We have decided to use an existing generator for use in the cloudy days so the 1.5 will now be zero.

I am still unclear on the battery providing all power or do the panels provide power for direct use and the excess charges the battery during the day and store only sufficient power for night. Batteries will have the 50% dod limit. We have determined about equal 12kW and 12kW split day and night.

[bcroe]

Batteries will not be active when other power is available, just stabilize the system. The batteries will
last longest if primarily on standby; charging and discharging them wears them out.

If you have grid power most of the time, it should be the cheapest power. Setting up to run on grid
with the batteries for backup should work. The grid could recharge them as well when it returns,
no need for solar. An extended outage would require the generator to recharge the batteries and
generally pick up the load. This is what the phone co has done for a century. Bruce Roe

[Sunking]

We have decided to use an existing generator for use in the cloudy days so the 1.5 will now be zero.

No sir than makes no sense whatsoever. That means you have no batteries.

When you say Autonomy is 3 days, and you use 24 Kwh/day means your battery capacity is 3 x 24 Kwh. Since you do no twant to ever discharge a battery more than 50%, a 3 day autonomy battery only gives you 1.5 days of power before you must shut down.

Here is the second issue. If you use 5 day Autonomy means you use 20% of the battery capacity per day. At 20% daily DOD the batteries can last up to 5 years. If you use 50% per day now you get less than 2 years.

To get the most out of your battery dollar is 5-day autonomy. That gives you roughly 3 cloudy days and minimum fuel burn cost. If you do it your way, I hope you have access to large amounts of cheap diesel fuel because you will be using a lot of it. That will cost you a lot more than batteries.

Look at it this way. A good 5 year battery with 5-day Autonomy will cost you roughly $0.60 to $0.80 per Kwh. How much fuel cost will it be for a Kwh?

[Sunking]

If you use 24 Kwh in a day, the panels must generate a minimum 36 Kwh.