OSENon
La Maison à Energie Minimum
Here a description of a domestic sanitary water installation through an electric water heater (accumulation)
, self-construction type, directly
powered by a photovoltaic solar panel without any electronics; this
installation operates satisfactorily since more than one year without any
troubles (of course, only when it’s sunny!).
The specificity of this installation is the simplicity of the design and
the absence of maintenance in order to warrantee investment profitability; the
electric current generated - 30V DC - is directly injected in the tank water
heater electric resistance, the tank being used as energy accumulator; none
electronics - none battery nor voltage regulator, none power supply - are
necessary. The system is simply resistive type (Joule effect) , fully static (none spare part under motion,
therefore no wear and no maintenance.
For profitability reasons, this is a mandatory requirement; indeed, for
a low power system (single panel type), electronics cost - even without
batteries - would be 5 to 10 time panel cost! Moreover, electronics being
subject to maintenance and scheduled replacement, this cost would completely annihilate
the profitability of the system, profitability being also an operation main
concern.
It is a national grid complementary installation only , who decrease
electricity costs ; but it’s not possible to substitute it for the national
grid , as there are not only pure resistive uses ( water heater or heating
purposes ) as domestics apparatus or lighting , and of course all uses during
no sun period ( 80 % of the time ).
Sanitary water
heating is the best use , as it’s used also in summer , most
sunny period , this insuring a good yearly
profitability ; also energy
storage in the tank – standard low cost electric water heater – allow
to store energy during the night without specific equipment .
Then we can define this installation is a Resistive storage Photovoltaic System.
For the design of such system only Ohm law knowledge is necessary; also
installation shall be designed under electrical design and safety rules.
The use of photovoltaic energy during all the year presents such
advantages:
-
No fluid used outside then no frost risks
-
Simplicity and reliability of the design
-
Best efficiency in winter ( under low temperature )
Averaged production is then not very different along the year as in
winter, efficiency is good but sunny hours are low, and in summer efficiency is
lower but sunny hours are higher.
For this document we will use such definitions:
Steatite: electrical resistor of a pressure tank being independent of
the tank (not in contact with heated water and resistor replacement requiring
not tank draining)
kWh: energy unit of an
1kW power apparatus in service during 1 hour ; for costs calculations we will
assume 15 Eurocent/ kWh (domestic fare, subscription charges excluded).
The used photovoltaic panel is used as a standard equipment generating
electricity with solar ; for the latitude - 47°Nord- it will produce during
1800 hours per year ; with 15.6% efficiency and solar radiation 1000 W/m2, we will recover for a 1,60 m2 panel
– south oriented (polycrystalline standard 60 cells 1640 x 990 mm) :
Panel power: 1000 W/m2 x 0.156% x 1.60m2 = 250 W then 0.25 kW
Recovered yearly energy: 0.22 kW x 1800 h x 0.70 = 277 kWh (assuming 70%
of max power, then 1260 kWh/kWc).
Yearly Production = 277 kWh x 0.15 €/kWh = 42 € (per panel)
Panel cost being 100 € we need then approx. 2 years for amortizing. Profitability
is then easy, if only panel cost is considered. Of course if electronics ( voltage regulator , power
supply , … etc. ) are used ( approx. 1000 € ) it is completely different, and
in that case no profitability would be possible even at plant start up ; maintenance of electronics
during equipment time life would also
increase total cost .
It is then required to use such design for minimum profitability for all
installation without investment grant.
Such designed installation, fully static type, would guarantee for a 25
year’s life time a good profitability (approx. 10 times panel cost recovered
over 25 years operation). It’s then a good bargain, and a friendly
environmental equipment producing power without any pollution.
We have to keep a record that electricity is charged with a lot of taxes
(approx. 70 %, refer to chapter 9 Taxes sur électricité), but taxes are a
help for profitability for a self-production installation. Without taxes, cost
of 1 kWh would be 0, 09 €/kWh and then a panel will recover 277x 0.09 = 25€ /
year instead of 42 € with all taxes.
Note:
1) If we consider water heater cost (200 € for 50L capacity) for profitability calculations ,of course the impact is important , but it
would be questionable to consider it , as
the heater tank can be considered as part of domestic installation and not part
solar installation ( whatever the installation is , heater tank is required ).
2) Current kWh cost is 0, 15€ but we can expect escalation during 25
years period.
3) 1800 h per year of sun, it’s average
4,9 h/ day
or 34,5 h / week.
For an optimized installation the panel shall be preferably rested as
follow:
-
South orientation
-
Out of any shadow impact (all the day )
-
With a slop 15° to 35° (35° being
the yearly optimum ) ; 15° slope is the optimum for winter
No shadow for the entire panel is a mandatory requirement as the panel
consisting in cells mounted in series, even if only one is under shadow all the
series will be impacted.
Installation at grade is also preferred for an easier electrical
connection and also for easier panel cleaning.
The water heater is a standard water heater; only it electrical resistor
shall be of steatite type in order to modify it; this is necessary for its
voltage adaptation: to be 30V instead of 230 V.
50 L capacity for one 220 W panel
(220W at selected operating
point) in order to optimize energy storage ; for a complete sunny day
water production – with water from 15°C ( summer conditions) to 45°C -will be
then :
0.220 x 3600 / 4.180 x (45-15) = 6.32 l/h ; then 7,9 hours as heating time for a tank 50 L is necessary ; if temperature
controller was settled at 65°C it could
be possible to store all the energy produced during 13,2 h of sun , this duration being approx. the max sun
duration of a summer day.
As a average value , sun duration is
about 4,9 h / day (1800 h / 365) involving an entirely free water production of 31 l / day ; for a week , it’s 217 liters.
Note : a water heater with a 50 L storage tank 50 L at 65°C
is equivalent to a 241 Ah electrical battery 12 V (2900 Wh).
The water heater is installed as close as possible of water consumers
for minimum heat losses ( refer to
chapitre 3 Chauffe-eau ); it could be
hydraulically independent , or used as a complement of a main water producer ( 230V
electrical water heater or boiler
) and connected in series and upstream .
The installation is strictly segregated from the domestic 230 V grid and
consists mainly in (for one panel 30V- 250W maxi- refers to Annex P1 Pictures):
-
A general switcher 2 poles ( for mini 20 A breaking )
-
A circuit breaker 2 poles ( for mini 20 A breaking )
-
A temperature controller dual type (control and safety functions); the
one’s provided with water heater should be acceptable if breaking capacity is sufficient.
A numeric voltmeter – wall mounting type – allowing operating conditions
checking ; an electronic resistor (1000 Ohm) is provided on power supply for high
voltage protection (as max panel voltage
is 36 V) and , the voltmeter being
powered by 4/30 V ; moreover this resistance avoid indication
blinking indication when voltage is
lower than mini required (lower
than 14V).
Cross section area of wires shall be 1.5 mm2 mini (copper) for a 250W
panel with a max distance panel / heater of and a max distance 5 m.
For any installation it’s required to check applicable rules and especially for different design - several
panels and /or different voltage or power - and to add necessary complementary
protection devices ( differential breaker , grounding system , breakers …. etc.
) depending on connection type ( series
, parallel ) ; refer also to link n°5 here bellow .
It’s the only specific item of the installation , as it’s necessary to
modify the resistance into a low voltage DC as supplied by the panel ; it’s
designed with (SS) stainless steel wire (0,5 mm2 c.s.a for main part , 2 mm2
for connections to wiring blocks) and with a length as required to design value .
.
Electrical laws as follow to be applied:
-
Ohm law U = R.I (U : voltage in Volt, R : résistance in Ohm, I :
intensity in A)
-
P = U. I = R. I^2 (P : power in W)
-
R = r .L / S (r :
resistivity in Ohm/mm2/m, L : length
in m, S : cross section area in
mm2)
These 3 laws are applied as per following sequence (refer also to annex
P2 résistance design):
1.
From the panel characteristic (annex
P3) operating point to be selected ( close
to maxi power ) , power and voltage to
be then read for characteristic curve (P
= U.I)
2.
Knowing P, U and I then
R is deducted (R= P / I^2)
3.
Knowing R wire diameter is
selected
The example as P2 Annex consider the conductivity ( r) at operating wire temperature , assumed 430°C
( radiating flux calculations out of this document purpose ) ; this have
an important impact on resistance design as SS resistivity is highly sensitive
with temperature.
You will note that for the resistor - main component - only 3 grams of SS is required ; even at 20€ / kg ,
cost is very low .
Important note: connections to wiring blocks shall be of higher section
- mini 2 mm2 - for avoid any high temperature at wiring points.
It’s sure that photovoltaic technology will improve performances of the
panels for next times ; with a same surface we can expect higher efficiency and
power ; then a present installation 1.6 m2 today , could be easily modifiable
into a higher power and then more profitable plant (panel replacement
expected within 25 years).
It will be also an important improvement if panels could operates with
light of the day instead of sun
radiations , as obviously period without
sun represents 79% of the total time ( 1800 hours / year with is 8760 hours ) ; electrical
production would be then 2.5 time more , we have only to wait and expect ….
Another way of improvement would be to add a sun tracker; it’s envisaged
at this time….
Selection of the operating point at max power – by design of resistance
value – could be also optimized in order to increase yearly production.
Also it’s not at all expected that taxes on electricity would decrease –
due to costs of nuclear wastes management for thousands of millions of
years??- and then this would increase
profitability of self-electrical production.
Maintenance costs are most probably higher than supposed at the date and
then such free maintenance installation - no electronics - would become then
more attractive.
Also this installation is within the “local producing philosophy” which
is under large development; vegetables could be locally produced but
electricity also, and if moreover as we can produce during the day and store
during night, what would be better?
As a conclusion this self- construction installation is also a very good
way to manage a simple electricity production; you could organize visit with
yours friends and envisage to write a book “the electricity without wastes
explained to my children “.
This installation will produce for next 30 years, not only a small
amount of money – without any public subsidies- but mainly it will produce
power without any wastes bequeath to generations to come.
Annex P1: Picture
Annex P2: Design of the résistance
Annex P3: Panel curves with
operating point
Some complementary links ( for information only) :
1-Production evaluating: http://www.photovoltaique.info/Estimer-la-production.html
2- Production evaluating: http://ines.solaire.free.fr/pvreseau_1.php
3-Solar data: https://www.sunearthtools.com/dp/tools/pos_sun.php?lang=fr (it’s a kind of
Google solar maps; enter your address in menu « user »)
4-Electric Safety Data :https://librairie.ademe.fr/energies-renouvelables-reseaux-et-stockage/3270-maitriser-le-risque-lie-aux-installations-photovoltaiques.html