Solar power for specific load in a building

Siraj_Safi

Hello everyone!

How to use Solar power for specific load in a building. Like in a hospital we have 200kw load but for Operation Theater we need about 60kw load we want to install solar system for Operation Theater to support the load of operation theater. Is it possible to make grid tied solar system for Operation Theater. Operation Theater has its own junction panel connected to the grid.

Thanks a lot.

BB.

Welcome to the forum Siraj,

This is a very difficult question to ask without more context... Why? (save money, save genset fuel, emergency backup power, full time power, go green, etc.).

Second... kWatts (1,000xWatts) is a "rate"--Like Kilometres per Hour... Yes, need to know the maximum power needed (like the speed limit on a road, 30 kph, 100 kph, etc.). But also need to know the quantity of energy needed (like 30 kph * 10 hours per day = 300 km per day).

Also need to know where this will be located... In Northern Germany, not much sun--In the winter, very little sun. In Karachi Pakistan, more sun (I am guessing your location as your IP address is "all over"--Kansas, France, Afghanistan, Pakistan using iplocation.net).

The reality is that off grid (battery powered) solar is very expensive (typically something like 5-10x more expensive than grid power; and equal to, or more expensive than generator fuel). Conservation is usually crucial to getting power needs low enough that solar can make sense.

I will do a quick sizing for you based on some guesses for a first pass estimate/give you an idea/back of the envelope design--That is a typical sizing for a full time off grid use (every day). You can adjust the calculations based on your true needs--not my guesses--It will still be the same math--Just different numbers.

First, 60 kWatts is a big load -- If this is a "rate" (peak loads)--That would be 250 Amps @ 240 VAC. Lets look at something slightly more reasonable (at least for solar). And guess that your requirement is 60 kWH spread over a 12 hour period (actual time in theater).

• 60 kWH per day (amount of energy per day) / 12 hours per day = 5 kW = 5,000 Watt load (rate--average "Watt" load over 10 hours) (lights, AC, medical monitoring equipment, etc.)
• 60 kWH (amount of energy per 10 hour day) * 1/0.85 AC inverter eff * 2 days of battery storage (night, no sun, bad weather) * 1/0.50 max discharge (longer battery life, another day of "emergency" power available) * 1/48 volt battery bank = 5,882 AH @ 48 volt battery bank

A 5,882 AH @ 48 volt battery bank is not small.. You can look for deep cycle batteries in your region for pricing. For example in the USA with good quality battery bank:

https://www.solar-electric.com/rolls-2-ys-62p-flooded-deep-cycle-battery.html

• 24x 2 volt @ 5,054 AH cells in series = 48 volt @ 5,054 AH battery bank (close enough for our quick calculations)
• 24x $1,900 flooded cell lead acid batteries = usd $45,600 worth
• 24x 258.5 kg/battery = 6,204 KG of battery weight

Next, solar panels... Two calculations needed. First is based on charging the battery bank--Larger battery bank, larger array needed.

• 5,054 AH battery bank * 59 volts charging * 1/0.77 solar panel+controller derating * 0.10 rate of charge = 38,725 Watt array nominal
• 5,054 AH battery bank * 59 volts charging * 1/0.77 solar panel+controller derating * 0.13 rate of charge = 50,343 Watt array "typical" cost effective maximum
  

Solar panels typically cost around usd $0.50 to $1.00 per Watt... And you will need to mount these on metal frames (another $1 per Watt???).

And sizing the solar array based on your amount of sun (hours per day, by season) and your loads. Guessing Karachi Pakistan, fixed array facing south:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Karachi
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 65° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.58	6.25	6.41	6.50	6.13	5.74
Jul	Aug	Sep	Oct	Nov	Dec
4.99	4.99	5.80	6.06	5.55	5.38

A very sunny area... Lets look at July with 4.99 hours of sun per day:

• 60,000 WH per day * 1/0.52 off grid AC system eff * 1/4.99 hours per day = 23,123 Watt array for July "break even"

Now typically, to minize genset usage, would suggest a 2x larger solar array to cover occational cloudy days, bad weather, etc:

• 23,123 Watt "break even array" * 2 = 46,246 watt array with "safety factor"

So~46,246 Watt minimum array would be suggested (based on my above guesses).

A 10,000 Watt AC inverter and something like a minimum of 10% rate of charge (5,054*0.1=501 Amps) for battery bank.

Compared with a 25 kWatt diesel genset (USA website) for (roughly) usd $10,000 plus ~48 liters of fuel per day:

http://www.centralmainediesel.com/order/Kubota-25-kW-Diesel-Generator.asp?page=KB2500

At 6,000 Watt load--Less than 1 gph (less than 4 liters per hour)--Or 12 hours per day around 12 gallons or 48 liters per day for diesel...

If this is for full time off grid usage--Maybe solar is better (cannot get fuel) for full time off grid use... If this was a "backup system" (few weeks or month or so of use per year), then a genset is usually more cost effective...

Anyway--My guesses... Your questions?

-Bill