M&V Solar Water Heating Guideline - Eskom

Measurement and Verification Standard Guideline for Low Pressure Solar Water Heating Systems v1r1
strategy of encouraging and assisting all sectors of South African society to reduce electricity
use and to relieve pressure on the electricity network. [1]
3 OVERVIEW OF RESIDENTIAL SOLAR WATER HEATING
TECHNOLOGIES
3.1 General
A conventional Solar Water Heating System operation is based on the physical
phenomenon known as natural convection or thermo siphon effect. This effect manifests in
circulating the water (or other designated heath transfer fluid) through the solar collector
and hot water storage tank. It is essential that the hot water storage tank is positioned
above the solar collector to use the natural flow of the water being heated. As water in the
collector heats, it becomes lighter and naturally rises into the tank above, thus creating a
circulation flow between collector and hot water storage tank.
To eliminate the dependence of the position of the storage tank and to increase the rate of
circulation /heat transfer a forced circulation may be employed, thus allowing the hot water
storage tank to be placed in a convenient place at the house.
3.2 Solar Collectors
3.2.1 Main Types of Collectors Used in South Africa
To obtain maximum utilisation of the solar irradiation various collectors are in use. The
collector performance characteristics are shown in Figure 1. and illustrate the efficiencies
of the main three types solar collectors use presently in South Africa, namely:
standard non selective-coated flat-plate collectors
selective coated flat-plate collectors
evacuated glass tube collectors.
From the graphs shown in Figure 1, it is evident that the different collectors should be used
for different purposes, namely:
The tested non-selective coated flat-plate glazed collectors perform poorly at high
operating temperatures. Their normal operational temperatures are in the limits of
30 – 80 o C, heat loss coefficient is above 5 W/m 2 .K, and optical efficiency of 0.8; thus
the applications for these collectors are limited for domestic hot water.
The tested selective coated flat-plate glazed collectors perform well at high
operating temperatures. Their normal operational temperatures are in the limits of
40 – 90 o C, heat loss coefficient is above 3 W/m 2 .K, and optical efficiency of 0.80.
Thus the applications of this collectors is extended for domestic and commercial hot
water large systems. The higher operational temperatures allow reducing the size
of the collector when compared with non-selective coated collectors.
8