A majority of current thermal rating programs require the equipment to be tested in accordance to a standard test under specified testing conditions. This approach provides reliable data because it is possible to replicate such tests within an accepted uncertainty band. There are, however, some rating programs which combine a standard test and a calculation procedure to produce a performance rating. Such is the case for the energy guide label for electric and gas hot water heaters. A similar method has been developed to provide a practical rating system with the goal of presenting an easily understood comparison between SDHW systems and conventional hot water systems. Note that the performance any individual commercial enterprise will experience may differ due to location and hot water usage. The thermal performance rating is based on the system design and performance projections derived from testing of the collector components used in the system, or from testing and evaluation of the system as a whole. The type of auxiliary system (e.g. gas or electric) utilized will have a large impact on the overall performance of the system. These differences arise because different types of auxiliary systems have varying standby losses and fuel conversion efficiencies. Although the auxiliary system may affect the solar system's performance, in many cases, the solar output is mostly independent of the auxiliary system used. Because gas backup systems have lower efficiencies and higher standby losses than do electric systems, it should be expected that the entire system's (including backup) performance will be lower, even if the solar output from both system types is equal.

Chapter 4: Comparison of Solar Thermal Systems with Conventional Systems

The Solar Energy Factor (SEF) was used in this study to calculate the performance rating for solar water heating systems as described further below. In this context, the SEF is calculated as being the amount of energy that is delivered by the system divided by the electrical or gas energy that is put into the system. The resulting calculation of the Solar Energy Factor is presented as a number that is comparable to the Energy Factor (EF) assigned to conventional water heaters by the Gas Appliance Manufacturers Association (GAMA); however, the exceptions noted in the Rating Parameters Section are also taken into account as shown below.

Where:

QDEL

Energy delivered to the hot water load: Using these rating conditions, this value is 43,302 kJ/day (41,045 Btu/day).

QAUX

Daily amount of energy used by the auxiliary water heater or backup element with a solar system operating, kJ/day (Btu/day). To convert to kWh, divide this value by 3,600 (3,412). To convert to therms, divide this value by 105,000 (100,000).

QPAR

Parasitic energy: Daily amounts of AC electrical energy used to power pumps, controllers, shutters, trackers, or any other item needed to operate the SDHW system, kJ/day (Btu/day). To convert to kWh, divide this value by 3,600 (3,412).

Source: OG-300 Certification of Solar Water heating Systems at http://www.solar-rating.org/facts/system_ratings.html#RATING

The Solar Energy Factor can then be converted to an equivalent Solar Fraction (SF) using the following steps:

For the standard electric auxiliary tank, the Energy Factor is 0.9; conversely, the EF for gas tanks is 0.6. The application of this method means that the Solar Fraction is the percentage of the total conventional hot water heating load (delivered energy and tank standby losses) provided by solar energy.

Notes:

1. An alternate definition for Solar Fraction is frequently used. In this alternate definition, solar fraction is the portion of the total water heating load (losses are NOT included) provided by solar energy.

2. The alternate method of calculating solar fraction will yield higher solar fractions. Therefore, researchers should use caution when comparing the solar fraction for specific systems, inputs into energy codes or outputs from f-chart applications to ensure that the same calculation procedure for solar fraction has been followed.

The Solar Energy Factor can be converted to an equivalent Solar Savings (QSOLAR) as follows:

Where:

QCONV

Daily amount of energy used by the auxiliary water heater or backup element without a solar system. The standard electric auxiliary tank has an energy usage of 47,865 kJ/day (45,369 Btu/day). The standard gas auxiliary tank has an energy usage of 72203 kJ/day (68,439 Btu/day).

EF

The Energy Factor is the ratio of delivered energy to input energy for the reference electric auxiliary tank without a solar contribution. The balance of the energy is lost to the surroundings due to standby losses and conversion efficiency.

QSOLAR

The Solar Savings is the amount of the total conventional water heating load (delivered energy and tank standby losses) provided by solar energy minus any parasitic energy use. To convert to kWh, divide this value by

3,600 (3,412).

Source: OG-300 Certification of Solar Water heating Systems at http://www.solar-rating.org/facts/system_ratings.html#RATING

Based on the foregoing, the Solar Savings calculation provides the amount of the total conventional hot water heating load (delivered energy and tank standby losses) provided by solar energy less any associated parasitic energy use.


The performance of the systems is determined from a computer simulation rather than by the actual test specified by the DOE procedure.

Source: OG-300 Certification of Solar Water heating Systems at http://www.solar-rating.org/facts/system_ratings.html#RATING

Table 1

Rating Parameters

RATING PARAMETER

(SI Units)

(I-P Units)

Environmental Temperature

19.7°C

67.5°F

Auxiliary Set Temperature

57.2°C

135°F

Water Mains Temperature

14.4°C

58°F

Total Energy Draw (QDEL)

43,302kJ

41,045Btu

Approximate Volume Draw

243l

64.3gal

Draw Rate

0.189l/s

3.0gpm

Draw Type: Energy

Number of Draws: 6 -- One at the beginning of each hour starting at 9:30 AM

Source: OG-300 Certification of Solar Water heating Systems at http://www.solar-rating.org/facts/system_ratings.html#RATING

Notes:

1. A comparison of different water heating systems can be achieved using the Energy Factor (EF) and the Solar Energy Factor (SEF); in addition, these calculations can be used to estimate average annual operating costs for the specified rating conditions.

2. The SEF includes all of the specified conditions for the DOE EF test, plus several solar specific conditions.

3. The EF and SEF can be used to compare solar and electric system's energy use on a one-to-one basis.

4. A higher SEF or EF indicates less conventional energy use, and consequently, lower operating cost.

A comparison of electric and solar and gas and solar thermal systems is presented in Table 2 below.

Table 2

Comparison of Electric and Solar and Gas and Solar Thermal Systems

Type of System

Calculations

Examples

Electric systems

Yearly Cost ($) = 365 days *12.03 kWh/EF*$x/kWh

Examples: (the following example assumes that electricity costs $0.12/kWh):

1. TYPICAL ELECTRIC WATER HEATER (EF = 0.86)

YEARLY COST = 365*12.03/0.86*0.12 = $612.69

2. TYPICAL SOLAR SYSTEM (SEF = 2.0)

YEARLY COST = 365*12.03/2.0*0.12 = $263.46

Notes:

1. The solar system saves $349.23 ($612.69 - $263.46) yearly.

2. This figure can be used as the energy cost savings basis for an economic analysis of a solar hot water system based on the assumptions for the standard DOE (EF) and SRCC-OG 300 rating conditions (SEF).

3. Other factors such as initial cost, maintenance, inflation, interest rate, and replacement costs also need to be considered when making an economic analysis.

Gas Auxiliary

Yearly Cost ($) = 365 days*0.4105/EF*$x/therm

Yearly Cost ($) = 365 days*0.4105/EF*$x/therm

Examples: (Assume that gas costs $1.60/therm)

TYPICAL GAS WATER HEATER (EF = 0.6)

YEARLY COST = 365*0.4105/0.6*1.60 = $399.55

TYPICAL SOLAR SYSTEM (SEF = 1.1)

YEARLY COST = 365*0.4105/1.1*1.60 = $217.94

Notes:

1. The solar system saves $181.61 ($399.55 - $217.94) per year.

2. This figure can be used as the energy cost savings basis for an economic analysis of a solar hot water system based on the assumptions for the standard DOE (EF) and SRCC-OG 300 rating conditions (SEF). 3. Other factors such as initial cost, maintenance, inflation, interest rate, and replacement costs also need to be considered when making an economic analysis.

Chapter 5: Discussion

Efficiency

One of the fundamental constraints to solar thermal system efficiency is the fact that just a fraction of the solar irradiance that falls on an individual solar cell can be converted to electric power (Fay & Golomb, 2002). Therefore, ensuring collection cells are placed at optimal angles for solar radiation collection represents an essential part of achieving maximum efficiency in these systems (Fay & Golomb, 2002). The energy production and energy savings that can be achieved through the use of solar thermal systems also depends on the efficiency of the type of system that is used for solar collection. For example, direct conversion is accomplished by…