Sizing off-grid systems in Ireland

 Hi,

I live in Dublin, Ireland. I am designing a off-grid photovoltaic
system for a remote home. While going through the solar insolation
data for Dublin, I found that there is a big difference in the
insolation level between summer and winter months. The worst month is
December and the solar insolation is for that month is only 0.465 KWh/
m2 (source: ISPRA website). The highest insolation month is June and
the corresponding solar insolation is 4770 KWh/m2 (source: ISPRA
website). generally the off-grid photovoltaic array is sized based on
worst month insolation. But if I size based on december month, it will
generate 10 times more energy in the month of June.

Please let me know how to optimize the sizing, so that it will give
sufficient energy to the load all round the year. Please also let me
know whether off-grid photovoltaic system is a economically viable
source of energy or not.

Regards.

Sid

 

Siddhartha wrote:


That's horribly low.



You need to size it to supply enough energy to feed the required loads
even under minimum insolation conditions. You have discovered the achilles
heel of PV solar in northern climes.

Graham

 On Fri, 5 Sep 2008 06:45:27 -0700 (PDT), Siddhartha


1. Minimize your electrical usage by all the energy saving methods you can
think of.  CFL's; energy efficient appliances; etc.

2.  There is no way around the fact that, if you are going to use the
property in December, you are going to have to have a system that will
provide the necessary amount of energy in December.  If you only have 0.5
kWh/m2/day during December, and you are using 5kWh/day, you will need at
least 10kW of panels (actually more once you figure in losses and
efficiency issues).

3.   You could consider other sources of electricity for this property --
e.g. wind, micro-hydro, fossil-fuel generator, etc.



You'll have to tell us what you mean by "economically viable".  Certainly,
you can compare the cost of grid-extension vs the cost of doing your own
system.  Add in the "cost of money" and you can see which one is less
expensive.

HOMER, a free program from www.nrel.gov/homer can compare the costs of
various systems, including all of the above and more, vs the cost of grid
extension.
--ron

 

Ron Rosenfeld wrote:


And don't forget the cloudy days. I'll bet that figure is an average of
something like 900Wh one day and 100 Wh the next day.



Wind's an obvious, esp if near the coast.



Viable would seem to me to depend on how deep the pockets are. As it doesn't
seem to be a new build from the OP's post, the hope of needing only 5kWh/day
seems remote. A multifuel stove (coal + wood) for space and water heating
would seem to be an essential.

Graham

 On Sat, 06 Sep 2008 05:56:20 +0100, Eeyore


Once I looked more closely at the data, I realized that the value the OP
was quoting probably referred to irradiance on a horizontal surface.

Insolation values, at least from the tables I've seen, do include cloudy
days.  They take an average for the time period -- usually over ten or more
years.

Using the NREL database, and the lat/long of Dublin, for the month of
December the variability seems to be from about 25-900 Wh, with 3-4 day
stretches close to the lower level.

The values given for insolation, at least in the NREL database, are for a
horizontal surface.  At Dublin's latitutde, with a panel tilted at the
angle of the latitude, the incident radiation on the solar panel is much
higher in December than is the global solar insolation (flat panel).

Running the simulations through HOMER, I find the following for December
(entire month) in Dublin:

Global Solar insolation (horizontal surface):
    16 kWh/m^2 for Dec (0.500 kWh/m^2/day)

With Panel tilted at latitude + 15°; with azimuth of true solar south:
    Incident Solar:    37 kWh/m^2 for Dec  (1.203 kWh/m^2/day)

Again, assuming a 5kWh/day energy use:

If we derate the panel to 80% to account for for such factors as soiling of
the panels, wiring losses, shading, snow cover, aging, and so on; figure
inverter efficiency of 90%, and include an appropriately sized battery bank
(11 days of autonomy, in this case); it seems we could "get by" with 6kWp
of solar panels.

Using a set of more basic spreadsheets supplied by BP solar, along with
their insolation maps, I come up with similar results, so that number is
probably correct, given the conditions described.

In the US, you could probably have such a system installed for about
$60,000(USD).

I don't know what the OP's energy use will be, or what installation costs
will be around Dublin.  But the costs should scale fairly directly with the
energy use, if he sticks with a pure PV system.

It seems to me that $60K USD is pretty expensive for 5kWh/day.

If he is using say 20kWh/day, and has good wind resource in the winter,
some kind of hybrid system might be preferable.  And he might be better
off, in that instance, with a different tilt on his panels (e.g. latitude -
15°).  He might also be able to get by with a relatively smaller battery
bank.
--ron