Solar System Using Grid for Peak Demand

Power & Energy Engineering general discussion Alternative Energy
#1

QUESTION
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).

  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

REPLIES

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.

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.

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=378162
Above is a snippet.

#2

A grid-connected solar system can be configured to prioritise the site’s own demand while controlling how much power is exported. For a factory drawing around 30 kW throughout daylight hours, the load profile should be compared with the expected PV generation before deciding on inverter and battery capacity.

An inverter with export-limiting capability can monitor the site’s grid connection point and reduce PV output when generation approaches the permitted export threshold. This can allow the system to use solar power on site without exceeding the network provider’s 5 kW export restriction.

The protection and metering arrangement also needs careful attention, particularly the local requirements for anti-islanding and grid connection.

For a commercial installation, I’d assess the load profile, PV capacity, export limit and connection requirements together. Stakelum Consultancy provides renewable-energy consultancy in Ireland, with work covering solar and commercial energy projects, so this type of assessment is relevant when establishing the practical system configuration.

With no significant night-time demand, batteries may only be worthwhile if PV generation regularly exceeds the factory’s daytime consumption.

#3

Hi Callum,
Welcome to SimpliEngineering.

I installed PV and a hybrid inverter a couple of years ago. The inverter can monitor net consumption and has a “zero export” mode, but I don’t use that. I set up the system architecture differently. Currently, it exports directly from the detached garage’s PV to my grid connection point. Shared at my grid connection point is the supply to my house, which is what consumes most of my electricity, of course. In the end, any energy my house consumes at the time the PV is producing and the inverter is exporting is never run through the meter. Thus, Ohm’s law does the work for me.

I had a power company willing to accept my system sized to produce as much as I consume, though. In the case of a utility company that imposes limits that don’t suit your needs, a deeper analysis of your consumption is needed.

The OP didn’t say what their daily consumption of energy is, just that it can peak at 30kW. Well my little house can peak at 30kW if I were to plug in every heater, the EV, turn on the oven and then run my welding unit all at the same time. They also didn’t say where on the planet they are located, which matters a lot. The use case isn’t complete, and anything from a 5kW to a 40kW system could be appropriate, depending on assumptions you choose.

#4

Another question comes to mind - a little off-topic though:

Has residential “plug-in” solar come to Ireland yet? I hear that the UK has passed a bill to permit that, but I think that might only extend to Northern Ireland, not the Republic.