Hybrid system linked to main grid

In general, solar electric power is either used or "lost". Batteries are the best, but imperfect, energy storage--And usually only good for 2-3 days of energy storage (and roughly 4-6x larger than your daily load--Expensive, especially if poor maintenance or other mistakes kill a battery bank early--Easy to do).

Or you have a small battery bank that can store a few hours of energy. Basically just long enough start the diesel genset. Or variations (i.e., enough storage to take you through a typical afternoon power cut, and only run the genset if you have a longer outage).

Feeding excess energy back to the grid (and turning your meter backwards) is good for the customer--But can cause issues with the if there are a lot of systems feeding power into the grid (more or less, if you have 10% or more of your customers feeding power back to the grid, it becomes very difficult for the utility to keep the voltage and frequency stable). Also, just turning the meter backwards, you are buying and selling power at retail--Sort of like buying food from the store in the morning and selling it back to them in the afternoon at the same price. The store cannot make money to keep the infrastructure going (transformers, distribution wiring, utility generators, etc.). This has been a huge problem for some regions in the US, Europe, and Australia. Lots of "solar and wind power", that has caused problems with over heated power lines and massive outages. In parts of the US, the regulators have basically made Grid Connected (and feeding power back to the grid) not worth the costs (i.e., meters that do not turn backwards, high monthly connection charges, etc.).

About the only thing you can do with "excess" PV power (if you cannot feed back to grid, charge batteries, etc.) is to find uses for the excess power. Such as water pumping/irrigation/pumping water to storage, electric hot water heating, etc. But even that has limitations (tanks/ponds are full, water tanks are hot).

You have to do some math and accounting to figure out what is the break even point between solar+batteries (plus risks of battery failure, maintenance) and running a genset (maintenance, fuel prices, etc.). So far, no magic solutions yet.

-Bill

If I understand correctly, the issue Alberto is seeing with internal power network problems (disconnecting inverters and generators) is just the small version of the problems the utility has with with Grid Tied Solar/Diesel genset connected customers.

Some sort of energy storage element is pretty much needed when mixing solar panels (which cannot supply surge power, and output varies with sun position+weather) and other energy sources (genset, battery storage, unreliable grid).

Designing the right size battery bank with the mix of energy sources is the solution... But limitations of Lead Acid battery banks, such as needing a pretty large battery bank to supply surge power and accept very high (short term) charging currents vs other battery technologies such as LiFePO4 which can be better in these applications vs much higher costs (and unknowns about long term reliability--Both batteries and battery suppliers/manufacturers).

Otherwise, conservation of energy usage (LED lighting, picking loads/devices/equipment to use the minimum amount of power needed to do "its job", load shedding of "optional loads" that don't need to run during grid failures, etc.) at least keeps the power plant smaller and cheaper.

-Bill

It sounds like a real difficult system to figure out and put right/make efficient.

Just to give you an idea of what can happen with a smaller, 3 phase generator+solar system:

http://forum.solar-electric.com/discussion/17986/is-the-system-set-up-right/p1

Having multiple sources of power and loads--And mix+match hardware--This will probably be a difficult task to set right--If not near impossible without abandoning some of the hardware.

More or less, "things" that are less than 1/10th of the source (i.e., if you have solar panels+GT inverter that is less than 1/10th of the local grid, loads, and/or generator output--You can probably ignore the smaller GT system. If the mix+match components (grid, genset, solar) are >1/2 between the "large and the small"--Then you you definitely have to take it all into account.

And you may find that some things are simply not compatible (i.e., a backup genset with Grid Tied Solar that is equal to, or more than the loads) and be more trouble than they are worth.

Out of curiosity--What is the goal of the various systems? Is it to save money (electrical/fuel costs) with solar, to run for many hours to days without utility power, save money on utility power with GT Solar, or what?

-Bill

In the US, there are regulations about medical devices (i.e., you cannot just go buy a cheap inverter and battery bank and throw it on the protected circuit for an operating room).

There are (at least) two ways to design a backup power system. Just large enough to run the loads for 15-30 minutes--And in the meantime, the genset is started, brought up to speed, and takes over the loads.

The other is to expect the batteries+inverter to run the loads for 1-3 days via a large battery bank. And the genset can either (or both) recharge the battery bank and support your loads (and/or solar charging too).

There are variations too... A Hybrid AC inverter--That has both a Grid Tie function (feeds energy back through the mains to turn the meter backwards), and can go "off grid" and support the loads with battery bank+solar+genset power.

There is even "generator support" in some of these hybrid inverters... Basically, you can run a (made up numbers) a 4 kWatt AC genset to power your loads, but the inverter will take AC power from the genset to recharge the battery bank (if the loads are less than 4kWatt), or if you have high loads (starting/short term loads) that exceed the Genset's 4kWatt output, the inverter will run "in parallel" with the genset and supply the extra power needed (i.e., 8 kWatt load, 4kW from the genset and 4 kW from the AC inverter+battery bank+solar array).

GT inverters, in general, are there to "save money" by slowing down the billing meter, and even turn the billing meter backwards (i.e., during the day, the solar array supplies 10 kWatts, with a 2 kW load--At night the grid supplies the 2 kWatt load). GT inverters are really a result of regulatory "green energy" or "green washing" by governments and environmentalists. What utility in the their right mind would want to sell you power at $0.20 per kWH and buy it back during the day at $0.20 per kW when it only costs them $0.05 to $0.10 to "buy power" from their generators (plus, how they pay for distribution wiring, transformers, and people to fix your power lines without "markup" of power costs).

If you are in a region that does not allow you to "sell power" back to the utility--It is very difficult to see how a GT system saves much money--And if you are running your own backup gensets--Management of the local AC backup grid becomes difficult (if GT solar output is >> than the AC loads, the line voltage / frequency goes out of regulation and knocks the GT inverter offline for ~5+ minutes--And or the genset gets power pushed backwards through it--Causing electrical problems, and/or unloading the diesel motor (diesels usually "like" around 40-60% minimum loading or they start having problems with "coking", oil pushed up past the piston rings, etc.).

When you have your own gensets (emergency backup power--GT solar inverters really can become a headache if the GT Power is >~10% of the total load/genset capabilities. There are GT inverters that can be "throttled"--But that just adds more complexity and may not be present in the current hospital power system.

And if you can save 10% of your daytime fuel (you still have to run the gensets for stable power, pure GT inverters do not set voltage/frequency of AC mains)--Is it worth the hassles and complexity?

It is an interesting set of issues--And could be a challenging project. However "challenging power project" + "main hospital power/backup system" usually do not go well together.

Since much of these issues revolve around the hardware (gensets, inverters, etc.)--Talking with the other stakeholders in the project (i.e., the guys that sold and maintain the equipment) to understand what is happening now and get some ideas about how to move forward--I would start there.

And, take everything with a grain of salt until proven otherwise--As the example of the link from Netherlands, even engineers get over their heads sometimes.

-Bill