A three-year research project using
a PV installation in an extensive, real-life, residential environment is now
under way at Fairfield University. Design and development studies will be
conducted using a grid-connected PV installation with a
set of 880 solar roof shingles covering a roof area of approximately 3,000
sq.ft. in a townhouse with eight student apartments. There are two merit
elements of note in this project, namely the size of the generating plant,
estimated at 14.5 kW, and the fact that it is grid-connected and integrated
into a living residential environment, providing power for the daily use of
most electric devices in this environment. Interfacing a “new”
power-generating system with daily residential activity, will allow us to
learn the high points and pitfalls in the traditional components of a PV
installation, and guide us to design more efficient systems. Engineering
studies will be augmented by studies on the ecological and financial aspects
of the project.
To
maximize the opportunity for experimentation and data collection, the solar
roof array, recently completed, has been segmented into three sub-arrays, each
one with its own controls, and each one feeding power to a different set of
student apartments.
Background and Objectives
The
present PV project is in two phases. Phase
I has been funded by a $100,000 grant from the W. M. Keck Foundation awarded
in January 2000, and a recent $10,000 grant from United Illuminating.
The power generator is a set of 880 solar roof shingles, each with a
generating power of 17 Watts, estimated to provide a total of approximately
14.5 kW. This will probably be one of the largest grid-connected PV
installations in use in a residential environment in the country. Phase I of
the project includes the design of the PV system, the purchase and
installation of the solar shingles in suitable arrays on the 3,000 sq.ft. roof
of townhouse #10 on the Fairfield campus, and connecting the shingles to
Combiner Boxes. These tasks are approaching completion.
In
addition to engineering design and development of the PV control system,
studies in the economic and ecological ramifications of the installation are
planned. The development team includes Engineering faculty, as well as faculty
from the Department of Economics and the School of Business, representatives
from the local electric power industry (United Illuminating) and the building
industry (PS Designs). Data loggers in the PV installation will continuously
monitor the system and accumulate data that will provide the basis for
engineering development, as well as for the economic assessment of the system.
Technical issues
The solar shingles for the Fairfield PV installation are manufactured by
the United Solar Systems Corp of Troy, Michigan. The core of the shingles is a
triple junction amorphous silicon (a-Si) alloy solar cell, each one composed
of three semiconductor junctions stacked on top of each other. Each shingle is
rated at 17 Watts. The National Renewable Energy Laboratory has been
evaluating these roofing modules in its Golden, Colorado, facility since 1993.
To facilitate engineering studies with the PV
installation on the roof of townhouse 10 on the Fairfield campus, the solar
array has been segmented into three sub-systems, each providing power to a
different set of residential units, i.e., units 1 and 2, units 3,4, and 5, and
units 6,7,and 8. Each subsystem has its own inverter, and each one can be
easily bypassed using the inverter-bypass switch and/or DC and AC disconnect
switches.
The roof exposure in townhouse 10 is
south-southwest, and it has a 270 tilt. This is about 150 less
than the 420 latitude in Connecticut.
Under these circumstances, the average solar radiation for flat-plate
collectors from month to month is as follows:
Only the first PV sub-system, providing power to
residential units 1 and 2, includes a battery pack for storing energy. This
system utilizes an SW5548 Trace inverter, while the other two sub-systems
include Omnion inverters. In the present configuration, the losses are
estimated to be 53% in the system with the Trace inverter and batteries, and
about 30% in the systems with inverters only and no battery packs. These
losses will be verified when the system is operational and will be the focus
of the planned engineering studies to reduce them appreciably.
The battery pack that will be used with the first sub-system consists of
24 2-Volt C&D CPV780 batteries. Power
from the solar arrays will be provided for all the appliances in the eight
residential units except for the electric stove and air conditioner. Also,
excess power from the array will be “net-metered”, i.e., fed into the
existing grid(3).
University records show that the average monthly
consumption of electric energy in each of the residential units is
approximately 500 kWh. With a
year round average irradiance of 4.4 kWh/m2/day, as shown in the
Table above, and taking into account the efficiency of the solar cells and
initial estimated energy losses in the system, it is calculated that each
residential unit will receive approximately 55-60% of the average consumed
energy from the PV solar array. It is expected that the proposed engineering
studies will measurably improve this number.
To facilitate data collection a
CR10X Data logger from Campbell Scientific will be utilized.