Hello from Roatan Island

[RoatanBill]

I'm a retired professional software developer with an electrical engineering degree, living on the island of Roatan. I worked for Con Ed in New York for several years and thermodynamically computer modeled their 13 power plants to instruct plant operators on how to generate the most power with the least fuel.

I'm in the process of evaluating using PV to run 3 to 4 A/C units at least part of the day on an array, with the rest of the day on utility power for a business I own. I don't own these units yet, and have no PV gear whatsoever.

Facts:
Temperatures are usually 85-95 degrees and the sun shines almost 365/year.
Insolation varies from 4.2 to 7.1 . 12 month average is 5.88 .
Utility power is expensive - $0.32/kWh, unreliable, and dirty since its generated with several large diesel gen sets.
Island utility capability is 16MW, but there must be thousands of small to rather large gen sets all over the island. There's no PV or wind turbines to speak of.
There is no net metering, and the utility won't allow any inverters to connect to what is euphemistically called a "grid".
A Texas billionaire has owned the power company for about 5 years and has made some minor improvements over the years, but electrical power here is a far cry from any city in the US. They're in the process of installing about 20 medium size wind turbines on a hill, but that project has languished for years.

Specifics:
The A/C units are high efficiency inverter models producing 24,000 BTU's (2 ton) at max output and 9,000 BTU's at minimum. Max power is 12.8A @ 240V and I have no idea what min power is. If you're unfamiliar with inverter compressors, they are continuously variable between min and max and are ON all day long varying their power demands according to the needed cooling load. They are not your typical full ON and full OFF compressors.

I've calculated that batteries for this project would cost a fortune, so I'm going to try to invent a hybrid design possibly using some batteries (and abusing them if necessary) and other equipment to transfer load between the PV side and the utility side.

[inetdog]

I'm a retired professional software developer with an electrical engineering degree, living on the island of Roatan. I worked for Con Ed in New York for several years and thermodynamically computer modeled their 13 power plants to instruct plant operators on how to generate the most power with the least fuel.

I'm in the process of evaluating using PV to run 3 to 4 A/C units at least part of the day on an array, with the rest of the day on utility power for a business I own. I don't own these units yet, and have no PV gear whatsoever.

Facts:
Temperatures are usually 85-95 degrees and the sun shines almost 365/year.
Insolation varies from 4.2 to 7.1 . 12 month average is 5.88 .
Utility power is expensive - $0.32/kWh, unreliable, and dirty since its generated with several large diesel gen sets.
Island utility capability is 16MW, but there must be thousands of small to rather large gen sets all over the island. There's no PV or wind turbines to speak of.
There is no net metering, and the utility won't allow any inverters to connect to what is euphemistically called a "grid".
A Texas billionaire has owned the power company for about 5 years and has made some minor improvements over the years, but electrical power here is a far cry from any city in the US. They're in the process of installing about 20 medium size wind turbines on a hill, but that project has languished for years.

Specifics:
The A/C units are high efficiency inverter models producing 24,000 BTU's (2 ton) at max output and 9,000 BTU's at minimum. Max power is 12.8A @ 240V and I have no idea what min power is. If you're unfamiliar with inverter compressors, they are continuously variable between min and max and are ON all day long varying their power demands according to the needed cooling load. They are not your typical full ON and full OFF compressors.

I've calculated that batteries for this project would cost a fortune, so I'm going to try to invent a hybrid design possibly using some batteries (and abusing them if necessary) and other equipment to transfer load between the PV side and the utility side.

Hello Bill and welcome to Solar Panel Talk! I am sure you will be getting helpful suggestions and questions. I have a few to start you off. but it would be most visible to other members if you repost your questions in the most suitable topics, such as off grid.


Well, first, I think you need to realize that totally independent of the scale and cost of your system a system based on battery storage will not be able to deliver power for your use at anything less that $.50 per kWh. And even that is a stretch figure reported by a member who is doing everything right and is counting on at least ten year battery life. More credible is $1.00 and up per kWh, especially if, as I expect, your equipment costs for stuff delivered on Roatan are higher than they would be in the US for the same gear.

With that out of the way for the moment, we can look at the intermittent and dirty power issue separately.

If you have a large enough array to match the power consumption during the day near solar noon you could have an off-grid system with a transfer switch and run off PV only when the sun is sufficient. The batteries would be primarily for starting surge and passing clouds so you could expect close to the shelf life of the battery bank rather than the cycle life.

For power outages, you would need the battery bank to be pretty large (enormous by typical forum member standards) and you could, I hope, be forced to rely on it relatively few times during the week. And perhaps only for loads other than A/C when the grid is out.

At the same time you could be running an AC input charger from the grid when it is present to fill in the shortfall of the panel production. It would essentially be a full-time dual conversion UPS with solar PV added to the battery charging mix.

If you have not run those scenarios yet, you should do so.

Remember when calculating your amortized cost for power to include periodic battery replacement, with the period much shorter if you take the battery bank below 80% SOC on a regular basis. Even during a power outage try to avoid going lower than 50% SOC and recharging rapidly from grid power as soon as it comes back.

[solar pete]

I'm a retired professional software developer with an electrical engineering degree, living on the island of Roatan. I worked for Con Ed in New York for several years and thermodynamically computer modeled their 13 power plants to instruct plant operators on how to generate the most power with the least fuel.

I'm in the process of evaluating using PV to run 3 to 4 A/C units at least part of the day on an array, with the rest of the day on utility power for a business I own. I don't own these units yet, and have no PV gear whatsoever.

Facts:
Temperatures are usually 85-95 degrees and the sun shines almost 365/year.
Insolation varies from 4.2 to 7.1 . 12 month average is 5.88 .
Utility power is expensive - $0.32/kWh, unreliable, and dirty since its generated with several large diesel gen sets.
Island utility capability is 16MW, but there must be thousands of small to rather large gen sets all over the island. There's no PV or wind turbines to speak of.
There is no net metering, and the utility won't allow any inverters to connect to what is euphemistically called a "grid".
A Texas billionaire has owned the power company for about 5 years and has made some minor improvements over the years, but electrical power here is a far cry from any city in the US. They're in the process of installing about 20 medium size wind turbines on a hill, but that project has languished for years.

Specifics:
The A/C units are high efficiency inverter models producing 24,000 BTU's (2 ton) at max output and 9,000 BTU's at minimum. Max power is 12.8A @ 240V and I have no idea what min power is. If you're unfamiliar with inverter compressors, they are continuously variable between min and max and are ON all day long varying their power demands according to the needed cooling load. They are not your typical full ON and full OFF compressors.

I've calculated that batteries for this project would cost a fortune, so I'm going to try to invent a hybrid design possibly using some batteries (and abusing them if necessary) and other equipment to transfer load between the PV side and the utility side.

Howdy RoatanBill and welcome to Solar Panel Talk

There is some good info and some very knowledgeable folks running around the place, might want to start with reading some of the stickys. We generally want to know how many kWh (kilowatt hours) the system needs to produce per day, it helpful to know max draw (start up) as well and dattime versus night time use, once we know these things I think there will be a few that will chime in to try to help out, cheers

[RoatanBill]

I posted that only as my intro, and intended to post specific questions in the appropriate sections as per your suggestion. I was formulating a post as your reply came in.

I've never seen any PV equipment up close and have no idea what gadgets are available and from which vendors. I've researched SMA equipment and like what I know about it so far.

As you indicated, the batteries drive the cost to unreasonable levels and I'll be trying to either eliminate them altogether or minimize their use strictly as a momentary UPS mechanism while load is transferred to the power utility.