All factors being equal, warm and sunny days produce the greatest “Btu harvest” from any solar-thermal collectors supplying a domestic water heating system.
The more intense the sunlight, the greater the heat input. The warmer the ambient air, the lower the heat losses from the collector enclosure.
Imagine a stretch of four warm and sunny days. Mix in the possibility that during this time the occupants of a home with a solar domestic water heating system might be away on vacation. Under such conditions, the only load on the system is likely to be standby heat loss from the storage tank. The result is a tank of very hot domestic water after perhaps the second day with a lot more sun yet to shine before hot water is needed at the fixtures. What happens now?
The answer depends on the system design. In many cases, the differential temperature controller operating the system can be programmed to turn off the collector circuit circulator(s) once the temperature of the storage tank reaches a user-set upper limit. At this point, the collectors enter a state commonly referred to as stagnation.
Zero efficiency
Every solar collector will eventually undergo stagnation when exposed to solar radiation but no fluid is passing through its absorber plate. This can occur for many reasons, including:
- Power outages;
- Collectors exposed to sunlight before they are piped and operating;
- Failure of a controller or associated temperature sensor; and
- Failure of the collector circulator.
Under stagnation conditions, the internal temperature of the collector rises until the rate of heat loss from the collector equals the rate of solar radiation absorber by the collector. The temperature of the absorber plate under stagnation conditions can be estimated by setting the instantaneous collector efficiency equation equal to zero — since the useful heat output during stagnation is zero — and solving for the collector’s inlet temperature. The result is given as Formula 1 (see in slideshow above).
Where:
Tstag = Estimated temperature of absorber plate during stagnation (ºF).
Y-Intercept = Intercept value from collector’s straight line instantaneous efficiency equation.
slope = Slope value from collector’s straight-line instantaneous efficiency equation (Btu/hr/ft2/ºF).
Tair = Ambient air temperature (ºF).
Here’s an example. Suppose a flat-plate collector has the following straight-line instantaneous efficiency equation: (see Formula 2 in slideshow above).
Find the stagnation temperature of the collector’s absorber plate, assuming the ambient air temperature is 85º F and the solar radiation intensity is 300 Btu/hr/ft2. The solution is as simple as plugging in the ambient conditions along with the slope and Y-intercept values: (see Formula 3 in slideshow above).
This is a very high temperature relative to normal operating conditions.
The materials used to construct any solar collector must be able to withstand temperatures associated with stagnation. Likewise, any fluid or vapor that remains within the collector under such conditions must be able to survive without accelerated chemical deterioration. This can be a challenge for some glycol-based antifreeze solutions.
A portion of the OG-100 certification standard issued by the Solar Rating and Certification Corp. requires collectors to undergo a minimum of 30 cumulative days of dry stagnation when total solar radiation in the plane of the collector is not less than 1,500 Btu/ft2/day. Following this exposure, the collector is disassembled and inspected for any signs of material degradation. Evidence of significant thermal degradation prevents the collector from being certified.
Protecting antifreeze fluids
Most current solar collectors on the U.S. market do meet the OG-100 standard and thus have proven they can withstand reasonable stagnation conditions. However, the same cannot be said about all the possible antifreeze fluids that may end up inside the absorber plate of a stagnating collector. Thus, it’s usually the antifreeze fluid rather than the collector itself that requires protection against stagnation.
There are several ways to protect the propylene glycol-based antifreeze solutions commonly used in solar domestic water heating systems from accelerated chemical degradation due to stagnation.
First, use a glycol specifically formulated for solar thermal applications. Such fluids typically contain higher amounts of stabilizers and inhibitors, which “fortify” the propylene glycol base against rapid drops in pH.
Second, incorporate a reliable means of heat dumping by which heat produced by the collectors can be transferred to a heat sink rather than being added to the storage tank or forcing the collector to linger at very high stagnation temperatures.
Heat dumps can be classified as either active or passive. Active heat dumps use one or more circulators or fans to create flow through the heat-dumping subassembly. Passive systems do not require any electrical power. Both approaches have strengths and limitations depending on the size of the collector array and other site-specific conditions. This month’s column focuses on active heat dumps. Passive heat dumps will be covered in a future column.
Most methods of active heat dumping require AC electrical power to operate circulators, valves and possibly fans. Given that a utility power outage on a hot, sunny afternoon is often the cause of stagnation, these methods should only be used in situations where one of the following conditions applies:
- The system is powered by a circuit supplied by an automatically started backup generator;
- The heat dump is powered by a photovoltaic module; or
- The heat dump is powered by an uninterruptible power supply.
The schematic in Figure 1 shows one approach that provides stagnation protection for the antifreeze solution in the collector circuit.
This system uses a small DC circulator powered by a small solar photovoltaic module. This circulator creates flow between the collector array and a heat sink, which in this schematic is assumed to be a length of fin-tube element.
The circulator and heat sink should be sized to dissipate the full thermal output of the collector array at a relatively high temperature, such as 200º. Be sure the flow rate through the heat sink will create turbulent flow rather than laminar flow. The latter, if allowed to occur, will cause a significant drop in convective heat transfer between the fluid and heat-sink element. To avoid laminar flow in a tube, select a flow rate that produces a corresponding Reynolds’ number of at least 2,500.
Figure 2 shows the electrical schematic associated with Figure 1.
During a power failure, or when the collector circulator is off, the spring-return diverter valve with 120 VAC actuator returns to its unpowered state in which flow can pass from the AB port to the B port. Relay (R1), with a 120 VAC coil, is wired to operate when the collector circulator is on. The normally closed contacts in this relay are thus open when the collector circulator is on, but closed when the collector circulator is off or during a power failure.
Relay (R2), also with a 120 VAC coil but wired to utility-supplied 120 VAC, closes its contact during a power failure. Thus, the circuit between the PV module and the DC circulator is completed only when the collector circulator is off (which is an assumed condition during a power failure) and utility-supplied power is off. Relay (R2) prevents the heat dump from undesired operation if the utility power is on, but the differential temperature controller has not yet turned on the collector circulator. If allowed to occur, such operation would dissipate heat from the collectors to the heat dump whenever the PV module provided sufficient power output to operate the DC circulator.
If the solar intensity is low, the power output from the PV module to the DC circulator also will be low and the flow/head produced by the DC circulator will be low. If the power output from the PV module is too low to operate the circulator, the stagnation condition likely will be of no consequence.
Depending on the solar controller used, it also may be possible to activate this heat dump subassembly when the storage tank reaches an upper temperature limit.
UPS at your service
Another type of active heat dump that protects antifreeze solutions from stagnation and the storage tank from excessively high temperature is shown in Figure 3.
This system uses a small 120 VAC heat-dump circulator powered by an uninterruptible power supply (UPS). When operating, the heat-dump circulator circulates the solar collector fluid through PEX tubing buried in a shaded exterior pavement, or perhaps directly in the earth. The amount of buried tubing required depends on the size of the collector array. The bypass valve allows mixing of the fluid coming from the collector array with fluid returning from the buried tubing circuits. This reduces the temperature of the fluid entering the buried tubing and thus protects it from rapid and wide temperature fluctuations.
The electrical controls for this system are shown in Figure 4.
The UPS always is plugged into utility power. The 120 VAC relay has a normally closed contact that remains open whenever utility power is available and closes when utility power is interrupted. At that point, a 120 VAC temperature setpoint controller is turned on. It measures the temperature of a sensor that is either located within a well on the absorber plate of one collector, or strapped close to a collector outlet pipe and well insulated.
When that sensor reaches a set temperature (200º as indicated in Figure 4), a normally open contact in the setpoint controller closes. This supplies 120 VAC from the UPS to operate the heat-dump circulator and diverter valve. They continue to operate until the collector sensor reaches some lower user-set temperature (100º as indicated in Figure 4) or until utility power is restored.
The combination of a small ECM circulator that operates on about 25 watts input power and a diverter valve requiring 5 watts input power could run for about five hours when supplied from a fully charged 1500 VA-rated UPS with a 168-W/hr. battery set (e.g., two 7AH / 12V batteries).
Figure 4 also shows a normally open isolated relay contact that would close if the storage tank reaches a set upper temperature. This isolated contact may be available on some differential temperature controllers. It allows the heat dump to function even when utility power is available, but the storage tank is at its upper temperature limit.
All antifreeze-protected solar thermal systems should be able to withstand stagnation conditions without creating accelerated degradation of the propylene glycol-based fluid. The methods described in this column are but two of several possible approaches. Future columns will present more alternatives.
