DogStar Solar's Alternative Energy Updates

Instantaneous Thermal Efficiency
December 23, 2010, 10:45 am
Filed under: Solar Thermal

Ref: Caleffi iDronics July 2009

The performance of any solar combisystem is implicitly

linked to the performance of its solar collector array.

The best solar combisystems are designed to enhance

collector efficiency. Doing so requires a fundamental

knowledge of what collector efficiency is and how it is

affected by operating conditions imposed by the balance

of the system.

In an “ideal” solar thermal system, none of the heat

produced by the auxiliary heat source would enter the

solar storage tank. This prevents the auxiliary heat source

from increasing the temperature of the storage tank above

what it would be based solely on solar energy input. Such

heating, if allowed to occur, delays the startup of the solar

collection cycle, and thus reduces the energy collected

during that cycle.

All solar tanks rely on temperature stratification to direct

heat added by the auxiliary heat source to the upper portion

of the storage tank. This minimizes heating of the lower

portion of the tank, and thus reduces interference with the solar collection control process.

The instantaneous thermal efficiency of a solar collector

is defined as the ratio of the heat transferred to the

fluid passing through the collector divided by the solar

radiation incident on the gross area of the collector, as

shown in figure 1.

Instantaneous collector efficiency can be measured by

recording the flow rate through the collector along with

simultaneous measurement of the collector’s inlet and

outlet temperature. The intensity of the solar radiation

striking the collector must also be measured. The Formula

can then be used to calculate the instantaneous

thermal efficiency of the collector.

Formula :


c = specific heat of fluid (Btu/lb/ºF)

D = density of fluid (lb/ft3)

f = flow rate (gallons per minute)

Tin = collector inlet temperature (ºF)

Tout = collector outlet temperature (ºF)

I = instantaneous solar radiation intensity (Btu/hr/ft2)

Agross = gross collector area (ft2)

8.01 = a unit conversion factor.

The phrase instantaneous collector efficiency can vary

from moment to moment depending on the operating

conditions. Do not assume that a given set of operating

conditions is “average” or “typical,” and

thus could be used to determine the

collector’s efficiency over a longer period

of time.

Instantaneous collector efficiency is very

dependent on the fluid temperature entering

the collector, as well as the temperature

surrounding it. It also depends on the

intensity of the solar radiation incident

upon the collector. This relationship is

shown in figure 2 for a typical flat plate

and evacuated tube collector.

The thermal efficiency of each collector is

plotted against the inlet fluid parameter. This

parameter combines the effects of inlet fluid

temperature, ambient air temperature and

solar radiation intensity into a single number.


Solar Thermal Heating
December 13, 2010, 7:48 pm
Filed under: Solar Thermal

Solar thermal heating is one of the most cost effective and efficient ways to incorporate the benefits of renewable energy into a building. Because the sun is used to generate heat, the fuel is not only clean, but it is also FREE!  That means lower utility bills! Once any higher initial costs of solar system equipment are recovered through avoided electricity costs, the system will only require expenditures for maintenance. Additionally, if a solar heating system is included in the mortgage of a new home, the cost savings and benefits are immediate. It is important to note that equipment and installation costs are significantly defrayed when utilizing federal, state and local incentives and rebates designed to encourage use of energy efficient and renewable energy technologies.

Solar thermal systems offer a range of applications to reduce energy costs. Solar heating systems are an excellent way to heat swimming pools, a building’s water, and interior space. A typical residential solar water-heating system reduces the need for conventional water heating by about two-thirds. It minimizes the expense of electricity or fossil fuel to heat the water and reduces associated environmental impacts (U.S. Department of Energy).

Solar heating systems are a clean energy technology that not only contribute to the health of the environment, but also protect human health and well-being. They are clean emitting, which means no harmful pollutants such as carbon dioxide, sulfur dioxide, nitrogen oxides, and other wastes are released into the atmosphere. The result is clean air to breathe and a healthy thriving planet for present and future generations.

These benefits all equate to a perfect outcome of lower energy costs, clean environment, improved conditions for human health and welfare, and increased energy security.