Power-up Solar Hot Water Systems

By Peter Biondo

Oventrop Commercial Solar Division

Solar hot water is a proven renewable energy technology that offers high-quality, reliable and cost-effective solutions to the greening of the hot water heating industry. There are hundreds of commercial solar hot water heating systems in service, complementing existing electric, natural gas, propane or oil water-heating services. Some of the very first solar hot water systems that I helped install in 1982 were large commercial installations for apartments, senior living complexes and recreation facilities.

The benefits in producing clean, renewable energy are its ability to substantially reduce operating costs, increase energy outputs and, with monitoring management, maximize the longevity of the hot water system. Applications for solar hot water include food and hospitality service, multi-family accommodation and recreation centers, particularly those with pools. The list also includes laundries, agricultural facilities, car washes, industrial processes and government buildings.

To continue the accelerated growth of the market, the challenge for the solar industry is to raise the level of expertise of the solar designer and the installation contractor and to have trained operation, monitoring and maintenance personnel for each commercial, industrial and large-scale residential project. This article will help you become acquainted with solar commercial hot water heating systems.

The Solar Ratings and Certification Corporation (SRCC) has some excellent documents to help you avoid typical design and installation errors. You can download them online at www.solar-rating.org. The "Active Solar Heating Systems Installation Manual," by ASHREA, is written for installation contractors of large commercial systems. The manual has a section that prepares you to bid for installation. The other documents include "Findings," "Conclusions," a "Recommendations Survey" and an "Operation and Maintenance Manual." These were written for solar hot water heating systems for Florida schools but contain valuable insights and lessons learned for installing large solar hot water heating projects.

A word of advice to experienced contractors in related fields (e.g., plumbing, mechanical, HVAC) who are new to solar heating: Be careful. It is different! The piping and related components are straightforward, but the roof work and controls can be deceptively difficult. Where contractors generally lose money or make mistakes is in the installation of racks, roof connections and collectors, and controls and sensors.

Also, watch out for items that are similar to those you are used to working with but just different enough to be tricky. For example:

You have installed tanks, but never any this large.

You have installed piping sloped to drain, but not collectors that must be sloped to drain.

You have installed setpoint controls, but not differential controls.

You have installed flow meters and thermometers, but not Btu meters.

You have filled a hydronic system with glycol, but have not charged a solar closed loop.

Lessons learned from the field

Collectors: Solar commercial hot water systems will be installed with either flat plate or evacuated tube collectors. In the United States, solar thermal collectors are tested and approved by the Solar Rating and Certification Corporation (SRCC). Each collector model tested receives the SRCC certification "OG-100," which is needed to qualify for commercial solar tax credits and rebates. To download the OG-100 rating for each collector model, go online at SRCC.

Something to consider for collector placement is accessibility. Ladders may need to be installed to make piping and collector arrays accessible on rooftops for easy service. Collector access should never be blocked. Drains must be installed and accessible to empty the collector and pipe work manually at every low point. A word of advice: Never use PEX pipe on collectors and associated solar loops. Solar loops can exceed the temperature ratings for PEX, and the pipe will fail.

Freeze Protection: Probably the single most important part of the system is the freeze protection that identifies the solar system. Solar thermal collectors transfer solar energy through a heat transfer fluid. Freezing occurs, for the most part, in just about every region of the country, so it is critical that the collectors be protected from freezing temperatures. The two primary design choices for freeze protection are closed loops or drainback systems.

In a closed loop system, freeze protection is provided by antifreeze, a propylene glycol mix that circulates through the collectors. The glycol loop is under pressure (25 - 35 psi) and all the air is removed upon initial charging and startup. The glycol picks up anywhere from 15 - 30 F passing through the collectors and is transferred to the solar storage tank through a heat exchanger. On the domestic-water side, the coolest water from the bottom of the solar storage tank is cycled through the heat exchanger and returned heated to the top of the storage tank. For multiple applications, the solar loop can be piped as a primary loop, with closely spaced tees for secondary branches to heat exchangers.

Solar closed loops should be protected from sitting idle when storage tanks are satisfied. It is necessary to install thermal heat dumps on solar closed loops to protect the collectors from overheating and to maintain a long life cycle for the glycol. To keep temperatures in the solar loop from approaching 250 F, fan coils can be installed for large commercial systems.

In drainback systems, the collectors remain empty (no fluid to freeze) unless solar energy is available. The working fluid, usually water or, sometimes, a glycol mix, is stored in a reservoir. While collecting solar energy, the water in the reservoir is pumped through the collectors and a heat exchanger for transfer to the solar storage tank(s). The collectors and piping are sloped to drain back into the reservoir when the system remains idle. The benefit to drainback design over closed loop is that there is no service required to maintain an antifreeze-pressurized loop, and, because an empty collector can sit in the sun, overheating protection is eliminated when collectors remain idle. With drainback, however, the collectors and all piping that is subject to freezing must be sloped to drain.

When collectors are mounted on rooftops, drainback is a workable option. Without a slope to drain, ground-mounted collectors usually have to be installed with closed loop. Once you have chosen either closed loop or drainback, there are other important considerations.

Common Errors: For large commercial systems, some of the more common performance problems occur as a result of unbalanced flow through the collectors and undersized heat exchangers. Another common error is the improper placement of the collector sensor.  It is important to install the sensor into the sensor well and insulate around the sensor well and then make sure that all piping is well insulated up to the sensor on the collector.

Balanced flow is important for maintaining high efficiencies of any hydronic heating system. For solar arrays, it is especially important to balance flow rates. Collector arrays may be piped in reverse return parallel. But with the cost of copper and labor for reverse return pipe work, it is recommended to use calibrated balancing valves for each array. Balancing valves should be set and locked by a trained technician commissioning the system. Generally, each collector requires a flow rate of 1gpm.

Heat exchangers for solar are sized for heat transfer at maximum collection and at a close temperature transfer exchange (close approach). Solar heat exchangers are sized differently from those used in typical DHW systems with boilers. Boiler loops run very hot to quickly raise the temperature of a domestic hot water tank. Solar loops, on the other hand, are running just above the temperature of the solar storage tank throughout the collection cycle.

Heat exchangers for solar are larger: Use the close approach chart for them. Close approach refers to the close temperature difference of the incoming solar fluid and the outgoing solar heated domestic hot water. If the heat exchanger is not sized large enough, the solar loop will cycle too hot, thereby lowering the efficiency of solar collection at the solar panels.

Solar Storage: Solar storage tanks for commercial projects are generally very large. They require volume to store the solar energy collected throughout the day. The general rule of thumb for solar storage sizing is that for every square foot of collector area, you will need to store anywhere between one and two gallons of water, depending on your location and the application. For instance, if you have an array of only eight collectors, you may need a 500-gallon storage tank. A larger system of 24 collectors may require a 1,500-gallon tank. The right amount of solar storage is important. Without enough solar storage, the solar tanks get too hot too soon, and the system shuts off. Too much solar storage and the solar tank won't get hot enough.

There are various methods of driving solar temperatures in tanks that reduce the need for auxiliary heat. First, to store and use solar hot water efficiently, the heat in the vessel should be stratified - the hottest water on top and the coldest below. Cold water feed is piped to the bottom of the tank, hot water to the service from the top. The water from the bottom of the tank is pumped into the solar heat exchanger and returned hot to the top of the tank.

Stratification is the norm for tall vertical tanks, but because of the sheer volume of these larger commercial systems, tanks may have to be mounted horizontally. There is a problem with using horizontal tanks for solar in that the temperatures will mix, lowering the effectiveness of stratification. This is noted to be a problem with commercial horizontal solar storage tanks. To solve this problem, these tanks can have a baffle welded mid-height across the length inside of the tank, so that the cold water can flow from one end of the tank to the other before mixing with the hottest water.

When piping multiple vertical tanks, configure the piping of the tanks in series rather than in parallel. When this is done, the solar heat is also piped in series, first into the last tank, which is the hot water outlet. This will create a temperature gradient between the tanks and, as a result, will widen the temperature difference from the hottest service hot water tank and the first tank fed with the cold feed.

The best method to boost temperature quickly in solar storage systems is to use diverter valves and controls to prioritize solar storage tanks. With this design strategy, large collector arrays can boost the temperature of one small tank quickly for fast recovery and use, then divert the remaining solar hot water into one tank or a series of tanks for storage. This allows for the highest temperatures to be generated quickly without losing the benefits of maximum thermal storage capacity. This is also a good storage method on cloudy days when a smaller tank volume is all that is needed to hold temperature.

Storage systems for solar space heating have a unique challenge: What to do with solar collectors that aren't being used all summer long? Without an additional summertime application for heating, such as a pool, the collectors do nothing. In some cases, a heat dump may need to be included in the design so that the collectors don't stagnate. The forefront of design for this application includes ground storage loops - either outside of the building in multiple thermal wells or under the insulated slab of a building in a deep bed of sand. This summertime storage method has been shown to work quite well to significantly increase the overall performance and savings of the solar space heating system.

Conclusion

By explaining these design principles, I hope to make you more familiar with the unique challenges of building solar hot water systems. These differences make solar heating a very unique trade. Because the technology is still very new and because good information is sometimes hard to find, common errors may be made by designers and installers. Attention to detail is important for the success of every commercial solar system. Good practices for design, installation, maintenance and monitoring will make every commercial solar hot water system stand out positively as the green alternative to hot water production. It is up to us.

Peter Biondo is Technical Sales Coordinator and Head of Design for Oventrop Commercial Solar Division. Peter has been involved in the solar heating industry over 20 years and has worked on over 3,500 solar heating systems nationwide. He carries a Mechanical Contractors License in the State of Arizona. He can be reached via e-mail at peter.biondo@oventrop.com.