GREEN SYSTEMS
to handle cold conditions or removing the fluid from the elements through the use of a drainback design. Anti-freeze is commonly mixed
with water to lower the freeze point and protect systems from damage associated with cold conditions. Propylene glycol is a standard product which is mixed with water and used in closed loop, pressurized solar thermal systems. The ratio of water and glycol can be adjusted to provide different levels of freeze protection (a 50/50 mix is standard). The glycol/water mix is separated from the potable water through a heat exchanger. The introduction of anti-freeze
and a heat exchanger to the system may have a negative impact on the efficiency.
If a heat exchanger is not
large enough or is not “solar friendly” this impact can be magnified. Since solar thermal systems operate best at low differential temperatures, the heat exchanger needs to be sized to work with relatively small differences in approach temperatures, sometimes as little as 10°F, and low flow rates when compared to boiler operations. One benefit of using closed,
pressurized systems is that there are no substantial piping requirements other than avoiding excessive fluid velocity and increased pressure drop. Also, the power consumption of the pumps can be substantially less than those of a drainback solar system since the pump is only overcoming pipe losses and not vertical lift. This can be especially important in commercial applications where panels may sit a significant height above the storage tanks. The biggest issue seen in anti-
freeze based systems is the degradation of system fluid over time. During periods of non-use, or in systems with undersized storage, it is possible for some collectors to sit hot on the roof and reach temperatures over 250°F. This causes some types of glycol to break down resulting in degraded freeze protection capabilities and raised pH levels. This can harm other system components and create the formation of “sludge” which can cause further issues in the system. In order to combat this issue many
manufacturers have begun to produce glycol mixes that are able to handle temperatures up to 350°F. The additives and inhibitors in these products help extend the life span of the anti-freeze, but most of the time the addition causes the fluid to no longer meet the FDA’s generally
regarded as safe (GRAS) guidelines. While still non-toxic, certain codes require that a double wall heat exchanger be used, adding cost and potentially lowering the systems efficiency. Other non-propylene glycol based fluids are becoming available that can withstand high temperatures (over 450°F) but issues such as high viscosity at lower temperatures can cause pumping issues. Systems that do not regularly
experience stagnation are in less danger of fluid degradation and may require minimal maintenance. If
commercial system that has high loads during the week and almost none over the weekend. For such systems it may be prudent to utilize a drainback method of freeze protection to avoid the issues associated with pressurized anti- freeze systems. A drainback system runs at low or
atmospheric pressure and uses water as the heat transfer fluid. This can help improve the system efficiency since water’s thermal properties are more ideal for use in transferring heat. A heat exchanger is still required since the pressurized
71
An example of a solar thermal system using an external heat exchanger. The preheated water leaving the tank is then distributed through a conventional water heating system. Certain valves and small components are not shown.
a system is designed properly with enough storage and is not oversized, closed loop systems can work well and run at high efficiency for years. However, it may sometimes be difficult to avoid such conditions and sometimes an oversized system is required. An example would be a system that is contributing to a heating load as well as hot water, or a
potable water system must be separated from the lower pressure solar system, so it is important that this heat exchanger be properly sized and designed. When the system is not operating
(no available solar energy and the tank is at maximum temperature) the fluid in the collector, supply and
e Continued on p 72
phc april 2011
www.phcnews.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84