More accurate sizing approaches

Consider a typical solar system providing heat to maintain domestic hot water at 140 °F (60 °C) in the domestic hot water storage tank. There is an heat exchanger between the solar system storage tank and domestic hot water system. To achieve a DHW temperature of 140 °F (60 °C), the incoming stored water temperature must be 5 °F (2.7 °C) warmer or 145 °F (63 °C) as it enters the heat exchanger, assuming a plate-and-frame type. The design temperature in the storage could be as warm as 15 °F (-9 °C) to provide a few degrees for temperature loss and extra storage capacity. There is another heat exchanger between the water storage system circulation and the collector fluid circulation, which means the collector fluid should be a hot as 160 °F (71 °C) entering the plate-and-frame heat exchanger.

Thus the solar collector leaving temperature will be a minimum 160 °F (71 °C), which will establish the collector efficiency and the amount of heating energy the collector will produce per sq ft area. Knowing the domestic hot water demands and the incoming cold water temperature this will establish the energy demands for heating the domestic water. The size of the solar collector can be determined from this information. In sizing the collector system, an important consideration in the design is the selection of peak solar radiation. As the peak value chosen is reduced closer to an average solar radiation value for a specific installation, there will be periods when the collector will generate heat that the DHC system cannot consume. At these times, the collector system will have no place to send the heat. The flow through the collector will stop and the collector system will go into stagnation with the temperature of the heat transfer fluid going above the design 160 °F (71 °C). If it goes high enough the heat transfer fluid will begin to vaporize.

In Section 4.3.1 (p 57) “rules of thumb” were used to determine the solar collector size. A more accurate evaluation for determining the size of a solar hot water system would use values specific to the collectors that are planned to be used in the system. The Solar Rating and Certification Corporation (SRCC) provides such Information and efficiency ratings for collectors available in the United States. This independent organization tests and provides a certification rating of various solar collectors and solar water heating systems. Equipment that has certified and rated by SRCC must show their certification label that provides the products performance rating.

The SRCC performance ratings in Btu/sq ft/day are provided under three different solar weather conditions – clear, mildly cloudy, and cloudy skies. Table 4.3 lists the five levels of service. The ratings also include durability and efficiency. This information can be used to compare different types of collectors. Although these ratings give the heat output for collectors, they do not provide cost information. Look at the amount of heat (BTUs) the collector delivers per day relative to its cost.

To compare two panels, first look at the service category (A through E) that represents water use. Then look at the output of the three solar weather conditions for each panel. To determine the collectors that produce the most heat for the least cost, figure the price per square foot of the panels by dividing the panel price by the panel area. When comparing panels, some may perform better in sunny conditions and some will perform better under cloudy conditions. Unless the system will be located in an area with lots of cloudy days such as the Pacific Northwest, it is more important for a collector to do better under sunny conditions than under cloudy conditions because there is more heat to capture on sunny and partly sunny days than on cloudy days.

SRCC also tests and provides performance ratings for small hot water systems. Information is provided on the storage tanks size and the total system performance for locations that can be selected from a pull-down list. The computer program TRNSYS is used to determine the values, which provide the Solar Energy factor (SEF), equivalent Solar Fraction, and equivalent solar savings (QSOLAR).

The performance of a solar hot water system (its “ability to capture solar radiation” and its “ability to deliver hot water”) depends on the configuration of the system (collector area, controller setting, storage volume etc.), the current state of the system (i.e., temperature of the hot water in the storage tank) and factors from its environment (notably the ambient temperature and insolation patterns over the year). Indicators (SF, SN, and SE) that are used to measure the performance of solar thermal systems are discussed in Section 3.1 (p 13). Another indicator is the standby hot water volume, which is the amount of water at the desired delivery temperature that the system can deliver at any time without any instant additional heating required for this delivery. With the following situation, the standby hot water volume is 83 (= 50x[65-15]/[45-15]) liters of water (22 gal) without any additional instant heating.

• Standby storage tank: 26 gal (100 L)
• Backup fraction: 13 gal (50 L)
• Set temperature of the backup: 149 °F (65 °C)
• Tapping temperature: 113 °F (45 °C)
• Cold water feed: 59 °F (15 °C).

The performance parameters SF, SN and standby hot water volume, together with the costs for the heat form a triangle; that by maximizing one, others may be affected negatively. For example:

• Maximizing for SF means that the collector area and the store volume accordingly will be maximized as to make more solar energy available, reducing SN, increasing the costs.
• Maximizing for SN means that the collector area will be undersized, because then excess available energy is avoided, but less solar energy will be available for the user reducing.
• Maximizing for standby hot water volume means that a larger fraction of the store will be at a higher temperature, which requires more auxiliary heating (lower SF) and also has a negative effect on the efficiency of the collectors.

Optimization thus involves a prioritizing and compromising (Figure 4.5).

Independent evaluation of single components of a solar system provides limited predictability of the performance of the system as a whole (i.e., yield). With larger components (in particular the collector area and the storage tank) the “principle of diminishing returns” applies. This means that although the yield will increase when the collector area or storage tank volume is increased, the marginal increase becomes smaller with increasing area and volume. In addition, maximizing one component over others also has a strongly diminishing effect. A diminishing effect usually also means an increase in energy costs (increase of marginal and average $/kWh). Simulations with various component sizes aid in the design of an (cost) optimal system.

There are design tools available for sizing the solar collector system. The National Renewable Energy Laboratory (NREL) developed the Federal Renewable Energy Screening Assistant (FRESA) software that can help facility managers determine if their building is a possible candidate for a solar water heating system. This Windowsbased software tool screens federal renewable energy projects for economic feasibility and evaluates renewable technologies including solar water heating systems, photovoltaic, and wind energy systems. The Federal Energy Management Program is developing a new version.

This computer program is available as a free download and it will model glazed, unglazed and evaporative cooling solar hot water collectors. It uses f-chart to calculate the estimated collector area required, annual solar energy yield, solar system efficiency, solar fraction and pumping energy needed. The analysis is based on the input of average monthly solar radiation, outdoor temperature and relative humidity and wind speed. The building’s hot water use is input for an average day over the year. This use cannot be varied. The results of this simulation compares favorably with other year-long evaluations. If a more detailed engineering and economic analysis is required, consider using the following software programs 

• F-CHART, correlation method, available from the University of Wisconsin
• TRNSYS, software, available from the University of Wisconsin

F-Chart is available for the purchase price of $400 for a single user. It has the capability of modeling flat plate, evacuated tubes, CPC and 1 or 2 axis tracking collectors. It can handle water storage heating, building storage heating, domestic water heating, and integral collector storage heating. The output can provide a life cycle analysis with cash flow.

Other computer programs that can be used for sizing the solar system are T*SOL and Velasoris Poly Sun. The Federal Energy Management Program (FEMP) Help Line (800-DOE-EREC) provides manuals and software for detailed economic evaluation and for the Energy Savings Performance Contracting Program, which allows federal facilities to repay contractors for solar water heating systems through bills for energy savings instead of paying for initial construction.