Solar system using grid for peak demand

[BigBill53]

Hey guys,
Does anyone know about an inverter for a solar system that will supply power to an installation, but if more power is required it gets it from the grid?

I am an electrical engineer, but I haven't had a lot to do with solar. Typically what I have seen is things like houses with solar panels, continually feeding power into the grid. But where I live the power authority can be hard to get on with. They won't approve anything over 5kW. I know some people who use around 30kW of power and would like a solar system.

They don't care about putting power back into the grid, they just want the solar to help reduce their power bills (they use most of their power during the day).

Thanks

 

[bacon4life]

Your first sentence sounds like a typical grid connected inverter setup. Is the issue that the power authority won't approve inverters rated above 5 kW? Or that the utility won't allow any backfeed?

 

[FreddyNurk]

There are quite a few inverters around that can integrate with solar, and depending on inverter and function, can also limit the export requirements.
Selectronic is one company that advertises this capability, Outback Power is another one. Depending on requirements and so on, batteries may be able to be integrated in order to balance power requirements and solar output as needed.

The local power authority generally has the consideration on whether or not to allow for connection to their network, along with certain standards for anti-islanding requirements and network protection. 30kW isn't really a small system for residential applications though.

 

[Desrod2]

Is the installation using DC? How do you power the installation at night?
In the typical installations I have seen, the inverter are injecting power at the AC connection point reducing the load or even back feeding into the network. The inverter is usually connected to the solar panels and convert the DC into usable AC. You might want to provide a single line diagram to explain because it is not clear how you feed your load and why you need to reverse the flow on your inverters.

 

[waross]

30 kW for a rsidence? Possible with about 25 tons of AC and other loads. Your actual lod may be much less than this. also rather than peak loads, consider the load profile, both over 24 hours and over the year.
Given that power exporting is not allowed, a 30 kW installation may be running at 50% or less capacity 80% or 90% bof the time. Once you find the "sweet spot" for sizing, greater capacity is often wasted money with a very very long payback time.
If you have already considered these factors then I appologize for raining on your parade.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigBill53]

Hey guys, sorry for being ambiguous (funny how it makes sense to yourself but then you explain it to other people and it doesn't come out clear!)

1) The network provider doesn't allow backfeed of any more than 5kW in this area. The site is grid connected.
2) This is for a very small factory that makes organic fertilizer (so not a residential load)
3) The current draw is around 30kW of power during daylight hours, and nothing at all during the night

 

[FreddyNurk]

There are means to get around the problem, although admittedly part of the problem is still getting the utility to accept the proposal.

If the inverter is capable of generating more than what the site requires as load, then it either needs another means to accept its output (say, batteries), or it needs to be able to reduce its output to suit both the site load and the utility requirements (which might be nil export to network, or 5kW export to network). I expect that a lot of the solar only inverter systems currently installed don't have that ability by themselves, if the network is present and within appropriate parameters, then they will output as much as they can.

Some solar inverters will have the capability to monitor the connection point (not all have the capability to do so through external instrumentation though!) and thus regulate the output based on available solar energy and grid requirements. There are also anti-islanding requirements for inverter systems that prevent the inverter from operating when the network disappears (UL1741, AS4777, other IEC standards).

In Australia some of the utilities have been more accepting of the issues in terms of connection of inverters to their network and the associated requirements, and will permit systems on the sort of scale mentioned by the OP subject to installation of appropriate protection schemes and/or use of standards compliant equipment (AS4777).

Its all possible, it might not be as cheap or easy as expected though, and subject to location and utility requirements, there may be equipment already available to do it.

 

[LionelHutz]

I took a look for such an inverter last year and didn't find any units that measured the utility connection so it could produce as much output as possible without backfeeding any power to the utility. All that I found were units that could limit their output to a set kW.

Around here, you have to grid tie a system through the government setup FIT program. So, you need a separate service so they can pay you for the power produced. This means you pay income taxes on this income and then pay all the taxes and other utility fees on the power you buy back. There are some advantages to this FIT program, but the government won't force utilities to also accept a net metering scheme where your system just offsets your site usage. The new smart meters took away the ability to just backfeed the old analog meter. So, I was looking to see if it was possible to do a smaller "guerrilla solar" install that could at least offset some of the site power demand during the day. But, I didn't find any suitable inverters for this.

 

[moltenmetal]

I'll be watching this carefully myself, for a much smaller residential application. I want to offset my grid demand whenever the sun is shining without backfeeding anything, ever...The cost to participate in the FIT program is higher than the benefit it would generate for my application because I have a very limited area available for panels- but those panels would definitely save me money by shedding peak summertime demand. So I need to do everything on my side of the meter, and never back-feed. The solutions I can come up with are all kludges- it would certainly be nicer to buy an inverter product which will do this for me.

 

[waross]

I've had an idea that I have never pursued, it may be worth looking into for your application.
Given that there is a good corelation between peak sunlight times and peak air conditioning times;
Will it be feasable to run an A/C unit with a VFD and feed solar derived energy directly to the DC bus?
The solar system will probably need a charge regulator that will limit the solar derived voltage to less than the DC bus over voltage alarm/trip.
Another variation may be to use an inverter to feed a sub panel and feed solar derived energy directly to the DC bus of the inverter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sbkenn]

Is there an economical way to run your machinery from DC ? I have found that inverters are quite delicate, and certainly don't like motor startup loads. Obviously, it is much easier to convert AC to DC, than tother way around. 300V DC is nasty stuff though, so high DC volts may not be appropriate. You might consider a rotary inverter too, which would be more robust though not as efficient.

 

[FreddyNurk]

LionelHutz, some of the Outback Power units I mentioned have the capability to do that now, the catch is that they require the inverter to effectively be the point of coupling to the utility, and the load (i.e. your house) is then run off the inverter with a bypass option should the inverter die. Not much use for 30kW as per the OP's request.

Some of the Selectronic units operate in the same way, but also have an option for external instrumentation so that they can separately monitor the utility connection, but I doubt that they're available for 60Hz markets.

I've not really seen any others that have that capability, and I'm not necessarily recommending either product mentioned above, rather they exist in the market (location dependent...) and do have those sorts of capabilities.

The DC bus concept is interesting, though I don't know of any of the smaller units that would suit, below a certain size they're almost all lower voltage DC (say, 48V) with some sort of step up capability (including transformer coupled) whereas I'd imagine most VFDs would have a DC bus higher than the mains supply. Some manufacturers are using either common DC bus equipment or Active Front End (half the VFD as waross suggested) for solar and energy storage applications, but none of them are of the size suitable for residential applications.

 

[LionelHutz]

FreddyNurk - are those not full battery bank system controllers? As you pointed out, they don't fit what is being requested.

 

[2dye4]

I have though a lot about how to utilize solar power without any possibility of backfeeding.

I have two scenarios so far.

1 An inverter powering an needed load that is near the solar output capacity. Air conditioning comes to mind. But with a fast transfer
switch that can power the load from either utility or inverter as requirements change. The transfer would be synced with the load
current zero crossing point as well as the inverter output being roughly in phase with the grid. So when a transfer is required the
control watches the inverter phase for when it syncs up with the grid then transfer on the zero crossing of the load current. The inverter
frequency would have to be slightly off of the grid so the system would not have to wait too long for a sync up.
The advantage is that an air conditioner could be started from the grid with the high inrush and run on the inverter when the energy is available.


2 Utility fed DC power supply ( power factor corrected ) that feeds the inverter with the solar system contributing the bulk when available. This
is simple but costly and inefficient.

Comments please ??