South or West for TOU (quantified) [RE-wrenches]

Tue Jun 15 12:01:00 PDT 2004

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Tony,

Thanks for providing this. Without doing the simulations, they generally
seem to make sense with a couple of exceptions. It is really hard for me to
believe that a 55 degree tilt facing south will outperform a 15 degree
facing south for TOU. I think this points to a possible error in the
calculations. Did you compare your analysis to Clean Power Estimator?

Next point is, as I have said many times, TOU is a double-edged sword. Even
with your best scenario, the overall value of the PV is only $0.185/kWh.
This is potentially less value than a customer that consumes 1000 kWh a
month and is on the tiered rate system. The tiered rate system is very
attractive for large users with PV systems that provide half of their
energy. TOU makes more sense for the energy conserving household. The other
side of the sword is that if you have air conditioning and live at home
during the day, it is almost impossible for TOU to be better than the tiered
system for PV.

It is not a simple answer for these questions, and the TOU rate structure
potentially exposes the contractor to billing disputes with their customers.
It is so much harder to prove that your system is doing its job when you
jumble in all the obfuscation related to the TOU rate structure. I have
always felt that the tiered structure is so much safer for the installer. If
the customer does a terrible job of energy conservation (although they swore
they never turn on the lights), the PV system only does better with the
tiered rate. That value can easily be over $0.20/kWh whereas you show that
the best possible situation for TOU is $0.185/kWh--perfect tilt angle and
all.

Thanks again Tony for the information--moral of the story is keep an open
mind to all the possibilities, but don't let your brain fall out. If you are
not certain whether TOU or tiered is better, stick with the tiered
structure.

Bill.

-----Original Message-----
From: Antony Tersol [mailto:tony at appliedsolarenergy.com]
Sent: Tuesday, June 15, 2004 8:09 AM
To: RE-wrenches at topica.com
Subject: RE: South or West for TOU (quantified) [RE-wrenches]

Short answer: South at 35 deg tilt is better than West at it's best tilt of
15
for San Francisco weather.  Even at 15 deg tilt, South is better.  But
south-southwest is best.

Long answer: For San Francisco, using real weather data, running a
simulation
that calculates performance over a 365 day year, every hour, for each
azimuth,
tilt combination (for each azimuth below, the simulation was done for 19
different tilts (every 5 degrees) x 365 days x 24 hours).

btw, this is weather dependent.  For example, for Fresno, straight south is
optimum for total production, as opposed to SF, which favors SSW.

  Site Latitude	(lat)	36.6   degrees

The following chart shows percentage of optimum at each azim/tilt combo.
For example, for west, a tilt of 15 degrees, produces 87.1% of optimum.
For south, a tilt of 35 degrees, produces 99.3% of optimum.
The optimum is at azimuth of 15 or 20 degrees west of south, at 35 deg tilt.

Sorry if the formatting is hard to read.  Anyone wishing a nicer format
email
me, and I'll send you a .pdf file.

I ran simulation with TOU value for each of the optimums, based on an array
of
18 sharp 165, sb2500 inverter.

azim \ tilt	0	5	10	15	20	25	30	35	40	45	50	55	60	65	70	75	80	85	90

ARRAY PERFORMANCE (%)

West
	-90deg	85.6	86.5	87.0	87.1	86.9	86.2	85.2	83.9	82.3	80.5	78.5	76.3	74.1
71.7	69.3	66.7	63.8	61.0	58.2

Optimum tilt

Total annual  energy / TOU value at 15 tilt (18 sharp 165, sb2500)
4040kwh $773.08	

Southwest
	-20deg	85.6	89.3	92.5	95.1	97.1	98.6	99.6	100.0	99.7	98.9	97.5	95.4	93.0
90.0	86.6	82.7	78.4	73.6	68.7
	-15deg	85.6	89.3	92.5	95.2	97.2	98.7	99.6	100.0	99.8	99.0	97.6	95.6	93.1
90.0	86.5	82.5	78.0	73.2	68.1
Total annual  energy / TOU value at 35 tilt (18 sharp 165,
sb2500)  		4618 kwh $856.49

South
	0deg	85.6	89.2	92.3	94.9	96.8	98.3	99.1	99.3	98.9	97.9	96.3	94.2	91.6	88.5
84.9	80.8	76.3	71.4	66.1
Total annual energy / TOU value	at 35 tilt (18 sharp 165, sb2500)
4588 kwh $837.23

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