|The PV plant is to provide 100% of the energy needed except . .
|Water heating will be provided by solar thermal collection.|
|Residence is all electric, no propane or natural gas.|
|Residence is new construction - no historical records available.|
|Expected energy demand derived from records at previous residence.|
|Previous residence used propane - conversion required.|
|Electric loads will remain the same.|
|Lifestyle and conservation habits have stabilized, however . . .|
|Occupants are in their senior years with escalating chill sensitivity.|
|The new house has 10% more floor area and 22% more volume.|
|The new house is better sealed and insulated.|
|Increased use of spot heating and space heating is expected.|
|Space heating and cooling by ground-source heat pump.|
|Mild summer climate, cool evenings, air conditioning rarely needed.|
|System designed for expansion should needs increase.|
Note: Non-infinite COP for solar water heating due to electrical pumping energy and
occasional backup by resistance heating.
|Forecasted annual electric consumption: 8,703 kWh
Determining the PV system capacity
The PV array will be supported on a semi-fixed structure with adjustable
elevation angle. Published long term photovoltaic capacity factors for
fixed-mount arrays in
the nearest city (San Diego) average about 0.21. This site is in the coastal
plain about 60 miles north of San Diego and is subject to a micro climate with
more late morning overcast and late afternoon fog. Furthermore, the acknowledged long-term accuracy of published capacity
factors is (+,-) 12%. For this exercise a worst-case capacity factor of 0.17 is
estimated. Based on this, the 8,703 kWh average annual demand will be properly
served by a PV system capacity of 5.84 kW.
Capacity = (8,703kWh/yr)/(0.17 x 8760hrs/yr) = 5.84 kW
The PV panels selected are Kyocera model KC-120-1 with PTC rating of 0.1057 kW
each. These panels can be expected to stabilize at about 0.0846 kW (80% of their
original capacity) after five years of operation.
The inverter selected is Trace model ST-2500 with peak efficiency
rating of 0.94%, however this efficiency is not always realized in practice. Performance curves of this inverter used in monitored systems seem to reveal that
a more realistic value.
Panels must be utilized in series-wired strings of 4. The lowest multiple of
4 that satisfies the design goal is 80 panels.
80 x .0846kw x 88% = 5.96 kW
A power level of 5.96 kW should satisfy an average annual demand of 8,876 kWh, which is adequate for the expected load.
0.17 x 8,760hr/yr) = 8,876 kWh/yr
Reconciliation with budget limitations
The installed cost of the 5.96 kW system exceeds the available budget, which
leads to the following process of rationalization:
Abandon the ground-source heat pump in favor of less predictable but much less
expensive air-source heat pump. The approximately $11,000 avoided cost can
be transferred to the PV system budget to support a 72-panel 5.36 kW system. This is a reasonable compromise for our mild climate sunbelt
region. In a
cold northern region, however, money might be better spent on the
energy-conserving ground-source heat pump instead of photovoltaics.
The pessimistic values used for capacity factor, inverter efficiency, and PV
output degradation may not in their aggregate represent reality. And some
advantage may be gained from the adjustable elevation feature of the array.
Producing surplus energy brings no benefit to the system purchaser. In
fact, since the power utility (SDG&E) charges a minimum billing for 365 kWh annually, an optimum system should incur an annual deficit of
this amount. The best approach is to err on the short side and design the system
for future expandability. Capacity can be increased if and when it is
determined to be necessary and funds are available.
It is therefore decided that the 5.36 kW system shall be expandable, consisting initially of 72
KC-120-1 PV panels and four ST2500 inverters. This initial system should
satisfy an annual demand of 7,982 kWh.
|(72 x .0846kW x 88%)(0.17 x 8,760hr/yr) = 7,982
Each inverter will be configured to serve five strings of four PV panels each.
Three of the inverters will be fully implemented, and the fourth
will serve three strings with provision to accept two additional strings if the
system is expanded.