New EE Checking In, and a Few Questions

retrodog · June 2009

Hi, all. Or maybe that would be Hi ya'all since I'm in Texas. I just ran across this site and it looks pretty interesting. Thought I'd join and check things out.

A few details about myself... I'm an electrical design engineer. I work on the International Space Station. Well I don't really work on it, but rather design things for it. I live in the Houston area (Friendswood) and have recently been getting more interested in Solar power. Oh yeah, I have a Vectrix electric motorcycle as well.

Anyway, most all of the design stuff is pretty straight forward, as I've been designing electrical circuitry and systems for over 26 years, but I just thought I'd ask a few questions...

I'd like to do a system for a dual type function. Primarily, it would serve as a backup power system for power outages (post hurricane and the like) and then potentially feed power back into the grid during regular times to get some slight payback. So I understand the need for batteries, charge controllers, inverters, and the like. But a few things I'd like to get info and opinions on are:

1. My local power company in the Houston area is Centerpoint. Does anybody know their stance on grid tied in systems, or who to call to get an explanation?

2. I have a nice roof over my back patio, facing SW and it would get a lot of almost direct sunlight. It's approximately 16'x14'. How useful would that space be, in terms of about how many KWHrs per day if fully covered?

3. I was thinking about starting out with a few 5-8 of the 200W panels. They run about $600-$700 each. Would that make sense for a start or some other size and configuration.

ETA. Oh yeah, here I am with my Vectrix. I might set up the system to charge the batteries on my bike cause I'm tired of people telling me that gas was burned to generate the electricity that my bike uses.

mike95490 · June 2009

Re: New EE Checking In, and a Few Questions

Welcome. One thing to prepare for, if you want to have PV as backup power for post-storm incidents, is to have a scheme for your PV panels to survive the lawn chairs and debris that's blown around by the storms. Kind of like putting plywood over you windows.
Also, only your "critical" loads should run off the system, unless you want to make it really large and expensive. Sometimes, the tax rebates, don't apply if you have batteries in the system.

BB. · June 2009

Re: New EE Checking In, and a Few Questions

Check this link out for Centerpoint's GT solar policy:

http://www.centerpointenergy.com/services/electricity/residential/generation/

It is difficult to "grow a solar electric RE system". In general, a system with ~3-3.5kW of solar panels is pretty cost effective to install (good size/priced inverter, building permit spread over entire installation cost, can get a solar installer interested in project).

There are hybrid systems that do Grid Tied while the sun is up and the utility is connected, and fall back to Off Grid when the utility power is down. Take a look at the Xantrex XW system for a start.

Grid Tie Solar Electric is the best bang for the buck ($$/kWhr costs). Grid Tied Hybrid systems next, and full off-grid the most expensive:

Very approximately:

~$0.10-$0.30 per kWhr for Grid Tied solar (assume 20-25 year life, little maintenance)
~$0.30-$0.45+ per kWhr for Grid Tied Hybrid solar (batteries make system expensive)
~$1.00-$2.00+ per kWhr for Off Grid power (Battery costs, less efficient because of extra components and battery cycling).

If you have not yet--your best investment is in conservation (insulation, double/triple pane windows, energy start appliances, CFL lighting, turning things off, new A/C system, etc.).

If your power outages are often / prolonged... Solar Hybrid is not a bad way to go (with generator backup). If your power outages are rare -- then setting up your home to run on minimum power with a (smallish/appropriately sized and efficient
AC genset) might be more cost efficient.

For my system, I have 20x 175 watt solar panels (3.5kW) that take up about 280 square feet of roof space.

16x14=224 square feet

(224 sq' / 280 sq') * 3.5kW = ~2.8 kWatts of solar panels.

You can use the PV Watts website to calculate what a such a system may generate (assume default values for GT Solar, 2.8 kW (SW=225 degrees true), Assuming you are around Austin TX:

[FONT=Fixedsys]Results

Month
Solar Radiation (kWh/m2/day)
AC Energy (kWh)
Energy Value ($)

1      3.92          251        24.35   
2      4.60          259        25.12   
3      5.37          333        32.30   
4      5.48          320        31.04   
5      5.68          338        32.79   
6      6.12          347        33.66   
7      6.33          365        35.41   
8      6.16          356        34.53   
9      5.62          323        31.33   
10     5.29          318        30.85   
11     4.22          250        24.25   
12     3.60          227        22.02   
======================================
Year   5.20          3687        357.64    [/FONT]

Now, most people in hot climates use ~1,000-2,500+ kWhrs per month in the summer--You can compare your monthly use vs what this size of system can supply.

One caveat... You said "mostly direct sunlight"... Solar panels (especially solar electric) need pretty much full sun. And shadows will dramatically reduce their power output while under shade (chimney, overhead power lines, etc.). Ideally, your panels should be shade free for ~9am-3pm at a minimum. In your case, also later in the afternoon since your roof faces SW.

-Bill

icarus · June 2009

Re: New EE Checking In, and a Few Questions

From a strictly $$ perspective, I would go 100% grid tie. It will be about 1/2 the price per ac watt of output,, and it will be way more efficient. I would then spring for a GOOD standby generator for my post hurricane needs,, ideally one powered by Nat. gas or propane,, or ideally a dual fuel so you can sort through fuel shortages after an event. The money saved by going with the genny will more than pay for the genny,, plus some more panels to boot.

Good luck and welcome to the forum. I am not going to embarrass myself with any electrical calcs for you!

Tony

niel · June 2009

Re: New EE Checking In, and a Few Questions

i don't see how being 100% gt will help him during a hurricane or any other outages. he needs the backup and wishes to sell too and that leaves a battery backed gt system. an allternative might be to use both straight gt and battery backed gt systems.

welcome retrodog and know that you will need to decide what is best for your needs and that includes the number of pvs. southwest may not be ideal, as one pointed south would be, but unless your afternoons are cloudier or tends to get rain more in the afternoons then it should be ok with some loss of collection. if that direction isn't good then you may want to consider another location for the pvs to aim them better to the south. this may be a pole or even some other ground mount arrangement. that would have need of some ground area in order to mount them, but may prove to be easier access for providing protection to the pvs during bad storms or hurricanes.
don't feel bad that it may have taken a fossil fuel to provide the power to your bike as those guys still are using bikes or vehicles that are using a fossil fuel. at least you are giving yourself some options.

icarus · June 2009

Re: New EE Checking In, and a Few Questions

Neil,

My point is (was) echoing Bill if memory serves,, that adding battery back up adds considerable layers of expense and complications: Including the purchase and life cycle cost of the batteries, charge controllers/chargers/hybrid inverters etc. If you are looking for security in being able to produce your own power no matter what, then that is a choice one can make. On the other hand, if your wish is to produce the maximum amount solar electricity net/net over a given time period,, then 100% grid tie is the way to go.

I am merely suggesting that foregoing the above added expense, buying a reasonable generator for the few days (hours/weeks?) of anticipated outages might make better financial sense. Only you can decide what is right for your,, but my feeling is having a substantial battery bank hanging around getting old 360 days a year, waiting for the 5 when it might be needed is not cost effective to me.

Just my opinion. Fact is I live off grid so any advice I give is filtered through that lens too.

Tony

niel · June 2009

Re: New EE Checking In, and a Few Questions

icuras,
no problem, but like as in my case i live close by many neighbors that would kill me if i ran my genny at certain times even if all power is off to them too. i just had an episode with an outage and could not fire up my genny as it was overnight(about 12hrs). my simple system was good for some lights, but did not do my frig any good and at the time we did not know when power would be restored as the outages were widespread county wide due to supercells that trained over the area. a tornado warning was issued, but i only saw the green sky with rotation in the clouds and no funnels or tornados. somebody did take a snapshot of a funnel, but it did not hit the ground.
point is that although generators are good at times, but for many of us it simply isn't the answer and every time somebody expresses an interest in a gt battery backed system that certain people always seem to diswade them from this good possible answer and i don't understand why. he specified hurricanes and other severe conditions that a generator may actually not have fuel for long or not be available. an ee would know what he's looking to do and why and no straight gt system is going to fit all of his requirements. i did mention in the intetrest of efficieny that running both systems could be feasable, but may fail to backup with enough power longterm.

RandomJoe · June 2009

Re: New EE Checking In, and a Few Questions

This is exactly the reason for my solar system, after spending a couple days without power in the middle of an ice storm. I was sorely tempted to go with a hybrid system myself, but after seeing all the red tape involved realized it wasn't worth the hassle. I didn't intend to install a system sufficiently large to run the whole house, just keep the refrigerator going when I was sleeping / at work, and keep some lights and radios on.

While there are times during normal days (like right now, in fact) where the batteries are charged and the extra power is "going to waste" instead of being fed to the "grid" to be consumed by other items running in my house, the quantity just isn't enough to make it worth the hassle and expense (the utility charges me a monthly fee for the pleasure) of going grid-tie. I'm considering the idea instead of some sort of transfer switch to automatically move some loads over to inverter when the bank is charged to use as a diversion load. That could also come in handy if the power goes out while I'm gone, the fridge could be automatically switched over.

It has occasionally been tempting to just go ahead and set up grid-tie "under the radar" since I'd be using all the approved devices anyway and in aggregate I would never be generating enough to go net positive. But on second thought I realized I could very likely wind up backfeeding at times, when I'm away at work during the day and nothing is running. Just my luck, the meter reader would come along while it's turning backward! :roll: Or, the more mundane - my meter is probably one of those that adds to the kWh used even if the power is flowing back to the grid - I'd be paying to produce!

I'm also a tinkerer and like the freedom of not having an inspected system. If I want to rearrange or try something else I don't have to worry about it.

Icarus, I first considered a whole-house generator, but don't need that much power when asleep / away at work and that seemed an inefficient waste of fuel. A small enough generator to just handle the refrig / furnace would be an awfully tempting target for thieves if left running when I'm not around to keep an eye on it... Not to mention potentially annoying the neighbors. Thus, I have an EU2000i to keep the furnace and other "nice-to-haves" going when I'm home and awake, as well as help recharge the battery bank if necessary. The solar system keeps the fridge, and other "need-to-haves" on while I'm at work or sleeping.

All that said, I do have a second "normal" use for the system, powering the ham shack so it's not like I installed something solely for backup power use. That made it a lot easier to decide to spend the money!

New EE Checking In, and a Few Questions

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