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
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%.
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.
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.
Description of Demonstration
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.   
Downloading and Running the Demonstration Program
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.
On-line Plotting of System Performance
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:
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)
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.
Plotting of Daily System Performance
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:
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.
Viewing Annual Performance Summary
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.
Using Hourly Data Output File
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:
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
A Note on Simulation Weather Data
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.
Contact Information
If you have questions about this demonstration or wish to inquire further about the TRNSYS program, please contact:
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