PV-SDHW SIMULATION
PV-POWERED SOLAR DOMESTIC HOT
WATER SYSTEM
- Table of Contents
- History and Background of
PV-SDHW Systems
- Description of
Demonstration
- Downloading the
Demonstration Program
- Installing and Running
the PV-SDHW Simulation Program
- On-line Plotting of
System Performance
- Plotting of Daily System
Performance
- Viewing Annual Performance
Summary
- Using Hourly Data Output
File
- A Note on Simulation
Weather Data
- Contact Information
- History and Background of
PV-SDHW Systems
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- The photovoltaic-powered solar domestic hot water
(PV-SDHW) system was patented in 1994 by A. Hunter Fanney
and Brian P. Dougherty of the National Institute of
Standards and Technology (NIST)
in Gaithersburg, MD. The PV-SDHW system is an
alternative to the conventional thermal SDHW
system in which solar energy is transferred to thermal
energy in a fluid circulated through an array of
collectors. The PV-SDHW system consists of a
photovoltaic array connected to several resistive heating
elements within a water storage tank. The PV array
produces electrical power during periods of solar
irradiation and this power is immediately dissipated in
the resistive elements. The system incorporates a
microprocessor controller to select the appropriate
combination of resistors to cause the PV array to operate
near its maximum power point (in terms of voltage and
current) during diurnal solar irradiation fluctuations.
The controller sequentially connects six resistive
elements to the array in parallel as irradiation
increases based on predetermined irradiance level switch
points. The resulting controller performance index
(CPI), or ratio of actual energy output to maximum
possible energy output from the array over time, is
greater than 95%.
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- PV-SDHW system has several potential advatages over
thermal SDHW technologies. The PV-SDHW system
eliminates the plumbing to and from the rooftop
collectors required in a thermal SDHW system. These
pipes connecting the collectors to a remote water storage
tankin a thermal system can be a significant source of
heat losses. The new system requires no heat
exchanger and antifreeze fluid for operation in cold
climates. These heat exchangers reduce the
efficiency of thermal SDHW systems. The
installation of a PV-SDHW system is simpler than for a
thermal system with no roof penetrations required.
System reliability is expected to be superior in
the absence of troublesome, failure-prone circulating
pumps, leaking pipes and fittings, and fouling heat
exchangers. The PV water heater design has no
moving parts to wear out or break down and makes no
noise.
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- The PV-SDHW system also possess a couple of key
disadvantages relative to the traditional thermal system.
Surely the most important drawback of the concept is the
high current cost of photovoltaic modules. Despite a 300%
reduction in PV system costs since 1982, PV modules are
sold at a rate of $5-7 per peak Watt in 1996. For systems
sized to meet the same load, the PV array of a PV-SDHW
system may cost two to three times the total cost of
purchasing and installing a thermal SDHW system. With the
PV market growing at 20% per year today, however, PV
module costs are expected to drop in the next decade.
Secondarily, the necessary surface area of the PV array
for a PV-SDHW system may be three to five times greater
than that of the collectors of a thermal SDHW system of
comparable performance. This larger area results
primarily from the lower solar energy conversion
efficiency of PV arrays in comparison to thermal
collectors. Installation of a PV-SDHW system is
complicated in the likely event that insufficient
unobstructed, properly-oriented roof area is available.
Improved PV module efficiency through continued research
in coming years could mitigate this problem.
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- Description of Demonstration
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- In 1995, NIST funded a research project at the University
of Wisconsin-Madison Solar Energy Lab to create accurate,
versatile computer simulation capability for PV-SDHW
systems in order to facilitate their design and analysis.
The project culminated in the M.S. Thesis
"Development and Analysis Tool for
Photovoltaic-Powered Solar Water Heating Systems"
(Paul M. Williams, UW-Madison, 1996). The TRNSYS
program was used to model PV-SDHW systems and the
resulting models were verified by comparison with
measured performance data from two prototype systems
operated by NIST: one at NIST in Gaithersburg, MD and one
at the Florida Solar Energy Center (FSEC) in Cocoa, FL.
Very good agreement was observed between
TRNSYS-predicted performance results and the measured
data. A generalized, user-friendly, user-formulated
version of the TRNSYS model of a dual-tank PV-SDHW system
is available here as a demonstration in the form of a
TRNSED simulation. In addition to producing a
complete range of outputs on the thermodynamic
performance of the simulated system, the model calculates
the estimated economic value of the PV-SDHW system to the
user. The model allows the user to select the
geographical location, tank volumes and energy factors,
PV array size and orientation, economic data, and other
parameters. For simplicity, the version provided
here for demonstration features just one PV module model
(the Siemens M55) and a choice of many previously-defined
sets of PV array, resistive elements, and irradiance
level switch points for their control.
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- Downloading and Running the
Demonstration Program
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- This TRNSED demonstration program is available as a
32-bit self-extracting, self-installing executable. To
run the downloaded program, the user must have an IBM
compatible computer (486 or higher) running Windows
95/98, 2000 or NT 4.0, a VGA monitor or SVGA monitor, and
about 5 MB of free disk space.
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- Click to download the
demonstration executable file UWPVSDHW.EXE.
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- On-line Plotting of System
Performance
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- If selected in the TRNSED input screen before the
simulation, an on-line display shows the values of
several system variables as the simulation progresses.
For example, the following are definitions of the system
variables plotted on-line during the TRNSED simulation of
the two-tank PV-SDHW system:
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- AUX_BOT - power delivery to bottom
resistive element in auxiliary tank (W)
- AUX_TOP - power delivery to top
resistive element in auxiliary tank (W)
- DRAWRATE - rate of water draw from the
system (kg/hr)
- PRE_BOT - power delivery to bottom
resistive elements in preheat tank system (W)
- PRE_TOP - power delivery to top
resistive elements in preheat tank system (W)
- TARRAY - photovoltaic array temperature
(deg C)
- TAVGAUX - average water storage
temperature in auxiliary tank (deg C)
- TAVGPRE - average water storage
temperature in preheat tank (deg C)
- TOUTSIDE - outdoor ambient air
temperature (deg C)
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- On-line plotting of each of the individual variables can
be turned on and off during the simulation by clicking
with the mouse button on the variable name at the top of
the on-line plot screen.
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- Plotting of Daily System
Performance
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- After a simulation is complete, the user can plot daily
totals/averages of several system variables for the
simulation period or any part thereof. The plots are
defined by the user through the TRNSED Plot menu. For
example, the following are definitions of the daily
system variables which can be plotted after a TRNSED
simulation of the two-tank PV-SDHW system:
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- AUXENERG - auxiliary electrical energy
used by system to heat water (kJ)
- CPI - controller performance index of
PV-SDHW system (energy output of PV array divided by
maximum possible energy output of array)
- ENERGSAV - auxiliary energy saved when
system is compared to standard electric water heater (kJ)
- HOTDRAW - mass of water drawn from
system (kg)
- SYSLOAD - system water heating load (kJ)
- SOLEFFIC - efficiency of solar device
(SOLENERG divided by TOTSOLAR)
- SOLENERG - energy added to water by
solar device (kJ)
- SOLFRACT - ratio of energy savings to
auxiliary energy use of standard electric water heater
- TOTENERG - total energy used by system
to heat water (kJ)
- TOTSOLAR - total solar energy which
impinged on array or collectors (kJ)
- The data which can be plotted through the TRNSED Plot
menu following simulations is also available in a text
file for possible use in other applications. The file
appears in the C:\PVSDHW\DECKS directory and has the .plt
extension.
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- Viewing Annual Performance
Summary
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- Following a complete one-year simulation of the PV-SDHW
system, a report is generated showing monthly and annual
system thermodynamic performance and economic
information. After the annual simulation is
complete, this report can be viewed through the
Windows/Output menu of TRNSED. The file viewed there is
located in the C:\PVSDHW\DECKS directory and has the .out
extension. If additional annual simulations are
run, the annual report for each is appended to the .out
file rather than deleting the file. This feature is handy
when running several simulations in which it is desired
to collect multiple reports for later viewing. To clear
the file, simply delete it the DOS or Windows
environment.
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- Using Hourly Data Output File
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- In the event that the user wishes to use ‘raw’
hourly system performance data for analysis in another
application, a data file is created after the completion
of the TRNSED simulation. This file is called pv2to.dat
for the two tank simulation (pv2t.trd). For
example, the following are definitions of the system
variables which are tabulated on an hourly basis during
the TRNSED simulation of the two-tank PV-SDHW system and
written to the file pv2to.dat:
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- LOADAUX - water heating load of
auxiliary tank (kJ)
- LOADPRE - water heating load of preheat
tank (or tank in one-tank system) (kJ)
- LOADTOT - water heating load of overall
system (kJ)
- LOSSAUX - heat loss from auxiliary tank
(kJ)
- LOSSPRE - heat loss from preheat tank
(or tank in one-tank system) (kJ)
- MOUT - mass of water drawn from system
(kg)
- TAVAUX - average water temperature in
auxiliary tank (deg C)
- TAVPRE - average water temperature in
preheat tank (deg C)
- TOTAUX - total auxiliary electrical
consumption of system (kJ)
- TOTPRE - energy addition to preheat tank
from PV array
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- A Note on Simulation
Weather Data
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- The PV-SDHW TRNSED simulation program is outfitted with
the TRNSYS Weather Data Generator. This component uses
monthly average values of variables such as solar
radiation and dry bulb temperature to develop an entire
year of typical data based on a set of statistical
algorithms. This component allows TRNSYS to be used for
any location for which standard yearly weather statistics
are known. The monthly average weather data used by the
Weather Data Generator in PV-SDHW TRNSED simulation come
from the National Renewable Energy Lab’s (NREL) National Solar
Radiation Data Base. It is based on 30 years of data
(1961-1990) for 239 U.S. cities.
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- Contact Information
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- If you have questions about this demonstration or wish to
inquire further about the TRNSYS program, please contact:
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- TRNSYS Coordinator
Solar Energy Laboratory
University of Wisconsin-Madison
Madison WI USA 53706
Phone: 608-263-1589
Fax: 608-262-8464
Internet: TRNSYS@sel.me.wisc.edu
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