Download PV Sizing Calculations for Harmony House Greywater System

A Stand-Alone Photovoltaic System for the Greywater System at the Harmony
House: System Design and Supply Lists
By Andy McDonald, Graduate Assistant, Robert A. Macoskey Center for Sustainable Systems
Education and Research
May 1, 2002
Introduction
A “living machine” greywater treatment system was installed at the Harmony House during the Fall
semester of 2001. The greywater system receives the wastewater from the sinks, washing machine,
showers, and dishwasher. The system requires the use of several air and water pumps to aerate water in
the tanks, and may require electricity for other functions. These may include an ultraviolet sterilization
device and laboratory equipment.
The designers of the greywater system wished to power it independently from the rest of the Harmony
House, utilizing electricity provided by a photovoltaic (PV) array. Funding in the amount of $1,700
was secured to purchase supplies for this purpose. The PV system was designed to meet the needs of
the system within the budget allotted. As the greywater system is experimental and subject to
modification and adaptation, the precise electrical load requirements at any given point in the future
could not be determined with precision. Therefore, the PV system was sized based on the current
pumps being used (which are small, low power, and AC), with consideration for budgetary limitations.
Design Considerations
The design process required us to make a number of assumptions and decisions about how the
greywater/PV system would be used. We planned for the pumps selected to operate 24 hours per day,
which significantly increased the overall load. While operating DC pumps would have enabled the
system to function more efficiently (converting power from DC to AC through an inverter entails an
efficiency loss of about 15%), we could not locate very low power, inexpensive DC water pumps.
Therefore, we designed the system to operate on AC power. (DC pumps could be added in the future if
an appropriate model were to be found).
PV system design requires you to estimate the number of Sun Hours per day available in your location.
This figure varies considerably based on whether you select minimum, average, or maximum weather
data, and also based on the tilt angle of the PV array. We chose to use average weather data, and plan
to adjust the tilt angle of the array 15o each season. This resulted in a Sun Hour figure= 3.8. By
choosing average weather data, the system may fail to produce sufficient power during extended
periods of very poor weather (from November to January, most likely).
We calculated the battery storage capacity needed based upon the assumption that the battery low
voltage disconnect level would be 50%. Battery manufacturers recommend discharging batteries to
only 50%- 80% maximum capacity. Choosing 50% as the maximum discharge will allow the system to
function longer during cloudy weather, meaning that we do not need as many batteries (i.e. as much
battery capacity). However, discharging batteries to 50% capacity could reduce the life of the battery.
The charge controller regulates the battery Low Voltage Disconnect (LVD) function. While some
charge controllers allow you to program the LVD level, our model (Pro Star 15) has a pre-set LVD of
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11.4 V (90% discharged, or 10% of the maximum capacity, according to Ed Mahoney of Concorde
Battery). This amount of discharge is substantially more than what the system was designed for. If the
batteries were drained to this level and not recharged quickly, they could be damaged and their lifetime
reduced. Mahoney strongly recommended against frequently draining batteries to this level. We should
investigate how to increase the LVD to 50% in order to protect the batteries and allow the system to
function according to its design. (See page 4 for calculations of system performance based on the
charge controller’s actual LVD.)
Based on an LVD of 50%, we also assumed that there will be a maximum of 4 consecutive days
without PV array charging. If (when) we exceed this number of overcast days, the system will shut
down. We need to plan for a backup when this occurs. The options include:
- Designing a grid intertie so that when the batteries shut off, the system begins to draw energy
from the grid.
- An alarm system of some kind that alerts house residents that the system has shut off,
prompting them to manually plug the load appliances into an outlet in the house.
- The load can be reduced to increase the length of time the batteries can go without being
recharged. We can calculate an estimate for how many days the batteries will last under a
certain load. If we find that the batteries are consistently drained down during the winter, we
can reduce the load, or add more batteries. The load can be reduced either by removing
appliances, or by reducing the amount of time that appliances are operating.
These design assumptions were discussed with a PV technician over the telephone. He ran our design
numbers through a computer model and projected that we would have at least two days each winter
when the system would fail to provide power (due to extended cloudy weather and limited battery
capacity). We chose to accept short-term system failure (and to learn to adapt to that), rather than
investing an extra $400 in another PV panel, or $150 in another battery.
This discussion illustrates the various assumptions and tradeoffs you need to make when designing a
PV system. We begin by analyzing our electrical load requirements and minimizing these loads. Then,
if cost were not an issue, we could over-design the system, assuming worst-case scenarios for solar
insolation, and choosing the optimum battery discharge level. In this way we could attempt to make the
system fail-safe. As our resources are not infinite, we chose to design a smaller system with the
understanding that it could fail during poor conditions. We will adjust our use of the system during
those times, substituting our own behavior changes for more equipment. We have this option with the
greywater system because we have alternative power sources (i.e. the grid) if the PV/battery system
fails. Additionally, maintaining electrical power within the greywater system is not vital, as it might be
in a hospital or laboratory. Settings with vital electrical needs would demand a power supply that was
fail-safe.
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PV Sizing Calculations for Harmony House Greywater System
LOAD SIZE:
Air pumps: 0.1 Amp x 12V= 1.2 W (x 4 pumps)= 4.8 W
Water Pumps: 0.5 Amps x 12 V= 6 W (x 2 pumps)= 12.0 W
Total Watts= 16.8 W
Weekly Power Demand
16.8 W x 24 hr/day x 7 day/week = 2822.4 WH/week
Inverter efficiency loss factor= 1.15
2822.4 WH/week x 1.15 = 3245.8 WH/week
Inverter DC input voltage = 12 V
Amp-H/week used by AC loads = 3245.8 WH/ week = 270.5 Amp-H/week
12 V
Amp-Hours/day:
270.5 Amp-H/week = 38.6 Amp-H/day
7 day/week
ARRAY SIZING
Battery Charge/Discharge Compensating Factor = 1.2
38.6 Amp-H/day x 1.2 = 46.4 Amp-H/day
SH = Average Sun Hours/ day in W. PA = 3.8*
Total Solar Array Amps = 46.4 Amp-H/day = 12.2 Amps
3.8 SH
Optimum Amps of Solar Module Used = 4.4 Amps
Total # of modules required = 12.2 Amps = 2.8 or 3 modules
4.4 Amps
BATTERY SIZING
(Amp-Hours/day) x (Maximum Number of Cloudy Days (Assumed)) =
38.6 Amp-H/day x 4 days = 154.4 Amp-H
Dividing above figure by 0.5 to maintain 50% battery reserve after deep discharge:
154.4 Amp-H = 309 Amp-H = max. amperage that will be drawn down for >10 minutes
0.5
Battery winter temperature multiplier, assuming minimum temperature 60 degrees F: 1.11
Battery Capacity = 309 Amp-H x 1.11 = 343 Amp-H
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SUMMARY OF LOAD, PV ARRAY, AND BATTERY DATA:
Load = 38.6 Amp-H/day
Constant Load = 1.6 Amp
Maximum load when all devices started at once= 3.0 Amps
Total Solar Array Amps= 12.2 Amps
Battery Capacity = 343 Amp-H
DC Voltage= 12 V
BATTERY PERFORMANCE BASED ON CHARGE CONTROLLER LOW VOLTAGE
DISCONNECT (LVD)- (The LVD of the actual charge controller is different than that used for
the system design calculations.)
Actual LVD= 90% discharged (0.90)
Battery Capacity @ 100 hour rate= 126 A-Hr
Total battery capacity= 126 A-Hr x 3= 378 A-Hr
378 A-Hr x .90 = 340 A- Hr = Actual capacity available to system
Design Load= 38.6 A-Hr/ day
Number of days system will run without re-charging= 340 A-Hr
= 8.8 days
38.6 A-Hr/day
Operating under these conditions, the system will function up to 8.8 days without any
battery recharge. However, the batteries will be very deeply drained by this point.
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Resources
Atlantic Solar Products- Vendors of renewable energy products.
9351-J Philadelphia Rd., PO Box 70060, Baltimore, MD 21237-6060
www.atlanticsolar.com, 410-686-2500
CET Solar- Creative Energy Technologies- Vendors of renewable energy products.
10 Main St., Summit, NY 12175
www.cetsolar.com, 888-305-0278
Creative Energy Technologies Design Guide and Catalog. 2002. Published by CET Solar. Provides
step-by-step guide to PV system design.
Concorde Battery, Inc.- Manufacturers of batteries designed for PV applications.
Ed Mahoney, Technical specialist regarding PV/battery systems.
909-944-2252
Ebara Solar, Inc.- Manufacturers of PV modules and water pumping systems. Located near Pittsburgh.
Dick Rosey, President- 724- 379-2001
Bob Stoehr, Engineer – 724-379-2034
www.ebarasolar.com
724-379-2000
Michal Vojtisek-Lom, Clean Air Technologies, Buffalo, NY. Design consultant for renewable energy
systems. 716-893-5800; email: [email protected]
Sandia National Laboratories, offers resources and design guidelines for PV systems.
www.sandia.gov/pv/
Sunwise Technology- Dealer of renewable energy products. Located in NY.
800-817-6527
Trojan Battery, Inc.- Manufacturers of batteries. 1-800-423-6569
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