Solar system using grid for peak demand
Solar system using grid for peak demand
(OP)
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
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





RE: Solar system using grid for peak demand
RE: Solar system using grid for peak demand
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.
RE: Solar system using grid for peak demand
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.
RE: Solar system using grid for peak demand
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
RE: Solar system using grid for peak demand
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
RE: Solar system using grid for peak demand
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.
RE: Solar system using grid for peak demand
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.
RE: Solar system using grid for peak demand
RE: Solar system using grid for peak demand
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
RE: Solar system using grid for peak demand
RE: Solar system using grid for peak demand
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.
RE: Solar system using grid for peak demand
RE: Solar system using grid for peak demand
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 ??