Still working through the list of questions and concerns raised by the proposal for rooftop solar.... Next Up? Why can't a guy (or a gal, or a group of guys/gals in any combination for that matter) use the electricity from the panels during a blackout?
This is not important. Really. I don't have medical equipment or other life critical loads. I just needed, way down deep in my "check to see if you are dealing with folks who really know their stuff" brain, to know why the electricity generated needs to go to the grid first and then come back rather than stop here at the house first if I need it and then join the grid if I don't need it. This conception, I hope you gathered from context, is wrong. It makes intuitive sense - but it can't work that way with current technology. I know that now (and will endeavor to explain it to you my kind reader in a moment.) But for now lets address the sales communication issue: the customer (me sitting with my checkbook) wants something, thinks it is possible and you cannot (nor can anyone else) deliver it. What do you do when faced with these questions? Well if you want one of those checks, you don't dodge the question, you address it.
Maybe I am a jerk (OK, I am certainly a jerk in some contexts) but, if you can't answer questions about how this stuff works (and does not work) you and your buddies don't get to put high voltage gear on my wooden house filled with my flammable family. For any price.
This question about why a photovoltaic array was not able to stand as backup power in emergencies was also a, "let me see if I (and you) understand (each other)" sort of question. I like that sort of question because I often uncover non-obvious misunderstandings this way. One just restates the thing. Does it still works in different words? Or does that restating flush-out some unstated exception, or remind the interviewee of some detail. Somewhere along the line I developed the habit and it has left me in very good stead.
Are you still wondering why we cannot use the electricity from the panels in a blackout? A couple of things make it too complicated and expensive and dangerous to do it. First off, the electricity needs to be converted to alternating current (AC) from the direct current that is created by the photovoltaic materials if it is to be used by your appliances or put on the grid. If you had some batteries (direct current) and/or some appliances that run on DC you'd be good to go in the blackout. But most of us don't get blackouts often enough to justify having all that expensive gear just for the once every few year blip in the power supply. So in the place of a battery (which can be thought of as a TIVO for energy - the energy comes when it does and comes out when you need it) you use the electrical grid. The grid functions as a super cheap battery if it is already available. One of the tricky bits in this "putting it onto the grid" business is timing the too and fro of the alternating pulse (at 60 hertz* in the US and other rates elsewhere) so that rather than cancelling out the energy you add to it. Grid-tie inverters do just this: the convert (invert) the DC to AC and pay out the AC at a rate that matches the frequency of the grid.
Imagine kids jumping a long shared rope. The rope has a pace and to get in and join the jump you'll need to watch and get a sense of it before you jump in or you'll mess it up for everybody. With the grid, you need the energy you add to the grid to alternate at the same rate and in the same phase as the larger system. In this way you can add to the energy stream. If you don't, you'll be a load for the system rather than a source. For this to work the inverter is built to be a follower of the dominant signal from the grid so it can hop in and join rather than go it alone. The danger in trying to lead rather than follow is when the grid is down... you end up "back feeding" the grid. When line-workers are out restoring power, the last thing they need is energy coming "back" from "down stream." Ouch.
The electrical grid and the wiring of your house are also based on a super abundance model. There are breakers and fuses to protect against too much power. But the case of too little power is weird. Unlike you body, which will reroute blood to vital organs at the cost of peripheral body parts in a shortage, the electrical system of your house will give every appliance as much as it asks for. Even if there is a shortage of electricity, as long as it is beneath the maximum limit of the circuit breaker each item that is turned on can take what it likes. This can lead to all kinds of squirrely (is so a word) behavior.
In theory, we could have house wiring systems that divvy up power for higher uses (like your kegerator or iron lung) first. And we do have some protections in place for hospitals - for instance the grid managers try to hold them out of rolling brownouts. OH and hospitals have generators too. But, in general, when the demand outstrips supply, there is a need to shut down parts of the grid to get demand below supply. The same is true for your house. But constantly watching the supply and shutting down circuits is expensive and complicated stuff for spaceships and submarines and such. Instead we just try to keep ahead of demand by predicting how much will be needed and in what increments it might spontaneously grow and where and then run the supply in anticipation of those needs.
In the commercial energy world (like running your big box retail store) there is something called demand priceing that measures the sorts of jumps in power demand you might put on to the grid all at once. They charge you to have that amount standing in reserve. It is expensive and has lead to some smart demand management systems that pay for themselves in short order as they save on these demand fees. Things like spinning up air-conditioning units one at a time or in small groups rather than all at once. Same with factory floor equipment.
Anyhow, standing alone as an energy producer is one task, participating in the grid is another, and jumping from one to the other on the fly is tricky (read expensive.) But not as expensive as having a customer stall out because you have not established trust.
So on to subsidy surfing...
*(named after Heinrich Rudolf Hertz - still more proof that all the good units of measure are taken!)
No comments:
Post a Comment