Innovations and Advances in Solar Thermal Systems


Inside this Article

Low-profile Solar Hot Water Collectors
Low-profile modules locate the headers along the long axis and mount in a landscape orientation.
Flexible plumbing interconnects
Flexible plumbing interconnects and proprietary mounting systems make installation of large systems easy and reliable.
Solar Skies SS16 square collectors
With their smaller size, Solar Skies SS16 square collectors are easier to lift to the roof than larger rectangular collectors.
Concealed connections on low-profile collectors mean less visual clutter.
Concealed connections on low-profile collectors mean less visual clutter.
Caleffi collectors
Caleffi collectors designed specifically for drainback systems have sloped internal headers, negating the need for angled mounting.
Solar Skies flexible solar storage tank
With collapsible sides, this Solar Skies flexible solar storage tank can be easily maneuvered through doorways and other tight spaces.
Taco’s line of solar circulator pumps
Taco’s line of solar circulator pumps offers features like variable speed, soft start, integrated controllers, and drainback-specific models.
Taco’s X-Pump Block
Taco’s X-Pump Block is a pump station with differential controller, two circulator pumps, and a built-in flat-plate heat exchanger.
Pump station (uncovered)
Pump stations integrate the differential controller, circulation pumps, check valves, priming valves, safety valves, temperature and pressure gauges, and other basic components into a neat, easy-to-install, and well-insulated package.
Pump station (covered)
Pump stations integrate the differential controller, circulation pumps, check valves, priming valves, safety valves, temperature and pressure gauges, and other basic components into a neat, easy-to-install, and well-insulated package.
Modern propylene glycol antifreeze
Modern propylene glycol antifreeze is not only nontoxic, but also high-heat resistant. Some bio-glycols are made from corn.
Differential controllers
The differential controller is the brains of a solar thermal system, monitoring system temperatures and activating pumps. Modern units are capable of handling complex systems and multiple scenarios, including overheating protection, diversion loads, nighttime radiant cooling, vacation modes, and multiple input and load monitoring.
Grundfos Vortex Flow Sensor
Some advanced controllers are capable of monitoring heat energy quantities based on temperature and flow with sensors like this Grundfos Vortex Flow Sensor.
Solar Hot Water kit
With the integration of SHW components becoming more standardized, some companies are offering SHW kits.
Low-profile Solar Hot Water Collectors
Flexible plumbing interconnects
Solar Skies SS16 square collectors
Concealed connections on low-profile collectors mean less visual clutter.
Caleffi collectors
Solar Skies flexible solar storage tank
Taco’s line of solar circulator pumps
Taco’s X-Pump Block
Pump station (uncovered)
Pump station (covered)
Modern propylene glycol antifreeze
Differential controllers
Grundfos Vortex Flow Sensor
Solar Hot Water kit

Changes in solar water heating have come in small increments. But when you add up all the recent minor innovations, you find that the world of SHW has changed significantly—mostly in serviceability, reliability, and installation ease. To get the industry’s viewpoint, we spoke with veterans like Bob Rohr, chief trainer for Caleffi; Randy Hagen, president of Solar Skies; and other industry insiders.

Innovative installation practices go hand in hand with innovative products. This focus is mostly on “things that work”—components and practices that offer a track record of improved performance, longevity, or ease of installation.


Flat-plate collector technology has not changed much. The best-performing black absorber plates are coated with selective surfaces, which maximize the collector’s absorption of short-wave solar radiation, and covered with a layer of low-iron, high-transmission tempered glass. The frames and insulation will withstand severe weather and up to 350°F inside without failure. Collectors are tested and rated by the Solar Rating & Certification Corporation (SRCC). Although there are many sizes and brands to choose from, collectors with similar construction tend to perform similarly.

There are increasingly more companies offering evacuated-tube collectors, which can be considered in colder, cloudier climates or where higher water temperatures are required. Flat-plate collectors usually cost less (per square foot of collector area) and perform well throughout most of the country. Collector manufacturers offer accessories like mounting hardware, union connections, and other features that can make an installation faster.

Low-profile collectors, which can fit more closely to the roof and have more concealed connective pipes and components, have less visual impact on the roof:

Despite requiring more solar collectors, use of these smaller square collectors can reduce labor. Ladders and rigging can be reduced or eliminated due to the small size and lighter weight (60 pounds). They are factory fitted with threaded unions to make the connection between headers with wrenches rather than a soldering torch.

Despite requiring more solar collectors, use of these smaller square collectors can reduce labor. Ladders and rigging can be reduced or eliminated due to the small size and lighter weight (60 pounds). They are factory fitted with threaded unions to make the connection between headers with wrenches rather than a soldering torch.

  • Solar Skies SS16 is a square (4- by 4-foot) flat-plate collector with a selective absorber coating. These collectors contain internal horizontal headers and vertical risers so that they connect side by side without external headers. This flow pattern is compatible with both closed-loop glycol and drainback systems.
    Despite requiring more solar collectors, use of these smaller square collectors can reduce labor. Ladders and rigging can be reduced or eliminated due to the small size and lighter weight (60 pounds). They are factory fitted with threaded unions to make the connection between headers with wrenches rather than a soldering torch.
  • The Heliodyne Gobi 404 is also a square collector that shares all the advantages of the Solar Skies SS16. It has a selective coating, includes O-ring threaded unions on the headers, and weighs 85 pounds.
  • The Viessmann Vitosol 100-F SH1B is a slightly larger low-profile collector. It has horizontal internal headers with a single serpentine riser tube. Because of the long, small-diameter tube, it is intended only for closed-loop glycol systems. The headers connect between collectors with a double O-ring plug using flexible stainless steel tubing instead of unions or couplers.

Drainback collectors. Caleffi’s new drainback flat-plate collector, the StarMax V, drains via internal pitched piping. The tilted headers drain to the middle of the header and then to an outlet in the center of the collector enclosure. Multiple collector center ports can be connected together, even with piping hidden beneath the roof, to drain an entire array. They can be installed parallel to roof lines instead of at an east-to-west slant, and with hidden piping, making them less obtrusive.

Evacuated-tube collectors were the most radical design innovation for solar heating since selective surfaces were introduced in the early 1980s. Inside each glass tube, the black selective surface heat absorber is a long, narrow strip surrounded by a vacuum. The vacuum is an excellent insulator (much like a Thermos bottle) that makes the tube reach and retain higher temperatures more easily. A fluid within the absorber conveys the heat to a manifold, typically located at the top of the tube, where glycol or water delivers the heat to the solar tank.

The individual components—manifolds, tubes, and mounting system—are relatively small and can be carried up a ladder easily. Historically, the price of these collectors has been higher than flat-plate collectors of equal size, but recent prices have been declining.

Storage Tanks

In-tank heat exchangers. When storing solar-heated water, pressurized water tanks with internal heat exchangers have proven themselves long-lived, with low or no maintenance. Pressurized tanks successfully used in the United States include:

  • Vaughn SEPCO SR “stone-lined” solar tanks available with a finned-tube coil and single- or double-wall heat exchanger;
  • Heat Transfer Products SuperStor Contender tanks with one or two single-wall, stainless-steel heat exchanger coils;
  • Triangle Tube Smart tank that uses a tank-in-a-tank, single-wall, stainless-steel heat exchanger design.

Unpressurized water storage tanks. There are also several options for unpressurized water storage tanks that are used typically with immersed heat exchanger coils, intended for drainback systems. These tanks are ventilated to the surrounding air through an open tube at the top to prevent pressure buildup in the tank.

  • Solar Skies’ Flexible Water Storage Tank, a collapsible atmospheric heat storage tank, is a new twist on the old idea of providing large water storage without using pressurized vessels. While this type of tank may require more maintenance compared with pressurized tanks, it has the advantages of having a much lower cost per gallon of storage capacity, easy transportability, and modular features. Besides being able to ship a tank with a very large volume in a very small package, collapsible tanks can come in handy in situations where a large tank is needed, but there’s not enough room to get it through a doorway.

Multifunction storage tanks. Many pressurized storage tanks include electric heating elements and a thermostat, much like a conventional electric water heater tank. This is the easiest way to provide backup to your solar-heated water. The electric element is usually mounted near the top of the tank so that only the top is heated electrically, and the bottom remains cooler for better solar heat gain.

  • The Vaughn tank and the HTP SuperStor have built-in electric backup. The Triangle Tube Smart Multi Energy tank offers the option of both an electric backup and a second heat exchanger coil for indirect heating with a boiler.
  • HTP’s Phoenix Solar Water Heater combines a high-efficiency boiler built into a stainless steel tank along with several heat exchangers. Solar heat can be delivered directly to the tank, and the 96% efficient burner can provide backup, rapid-recovery hot water.

Pumps & Pump Stations

Multispeed and variable-speed AC pump control. The solar heating industry has been packing more combined controls and features into its products. The heart of any active solar water heater is the circulator pump, which is becoming smarter and more efficient.

  • The Taco “00” series has controls built into the electrical junction box on the pump motor. The OO-VS control can vary the pump speed in response to temperature changes measured by a thermistor wired to the control. The 00-VT control includes a differential thermostat using two sensors controlling the pump’s start, stop, and speed. There are a number of other useful versions in this series of pumps and they all are programmable to tune the pump’s performance to the job.

Solar pump stations. Solar domestic hot water systems in the United States have adopted the European-style enclosed solar pump station, which typically includes a circulating pump for the collectors, check valves, isolation valves, gauges, and are insulated with molded foam. Some include a differential control to activate the pump based on tank and collector temperatures.

  • Many solar pump modules are designed for single-pump systems, such as the Caleffi 255 series, which can have an AC or PV DC circulator. Taco’s X-Pump includes a flat-plate heat exchanger and two circulating pumps—one for the solar collectors and one for potable water.
  • For combination space and domestic water heating systems, as opposed to solar domestic hot water systems, PAW, a German hydronic manufacturer, supplies modular piping systems that include “plug-together” manifolds and virtually all of the other heating system components. This makes a system easy to lay out and install. Pump modules are available with thermostatic mixing valves to control output temperature.

Some solar suppliers also package complete pre-engineered solar water heater kits with all of the necessary components, including the collectors, pump station, pipes, expansion tank, and glycol, taking the guesswork out of an installation.

Heat-Transfer Fluids

Propylene glycol & water. In drainback systems, water is usually the heat transfer fluid. During cold and sunless periods, the water drains out of the collectors to prevent freezing. For collectors that cannot be drained, the most common heat transfer fluid is propylene glycol (PG). It has a long track record and is widely available at a reasonable cost. This is not automotive antifreeze, which is ethylene glycol and is toxic—it should not be used in domestic solar heating equipment. Solar heat transfer fluid is usually a mixture of 50% to 60% water with less than 5% additives to improve its chemical stability.

Not all PG products are the same. The most important consideration is the manufacturer’s high-limit temperature rating. Some products can operate normally up to 350°F. Solar collectors can exceed these temperatures during stagnation conditions, so the overheat prevention system must be designed with the PG temperature limit in mind. When in doubt, follow the recommendations of the solar equipment supplier.

  • Solar glycol products include DowFrost, CryoTek, and Tyfocor.
  • Glycol made from renewable resources has been in development. Bio-glycol, usually made from corn, is a high-quality product with superior heat transfer capability, potentially better price stability, and a high temperature tolerance. Dynalene, for example, has a high temperature rating of 350°F. Bio-glycol is also available from DuPont and other manufacturers.


Differential controls. A solar water heating or combination system’s efficiency and provided comfort is only as good as its control system. The differential temperature (DT) control is the brain of an active solar water heater. The basic function of a DT controller is to measure two temperatures, turning the system on when one measures higher than the other. Modern DT units offer a low-limit setting to prevent operation until a minimum temperature is reached, and a high-limit setting to prevent overheating. Some include a vacation mode that allows heat shedding at night, and some have sensors and functions for energy measurement and additional control jobs.

  • Basic AC-powered, single DT controllers for simple solar water heaters include IMC Instruments’ Eagle 2, Steca TR0301U, Tekmar 156, and ReSol DeltaSol AL. For PV-powered DC solar heat control, the ArtTec DTC (1, AT, or D) and the Eagle D2 are available.

Data logging & monitoring. A trend in solar controls is toward more comprehensive systems, where one control box handles all the sensors and pumps.

  • The latest models, such as the Steca TR 0603mc or the ReSol DeltaSol BX, have more of this capability but require expertise to program the control sequences that are right for each job.

Many controllers offer energy measurement or estimating that can be recorded and displayed.

Btu measurement. In the future, we may see renewable energy credits (RECs) for solar heating. Solar heat can be metered to calculate a value for the fuel saved and carbon emissions avoided. If these RECs take off, accurate energy measurements will be important. Also, when solar heat is provided to a commercial tenant, the value of that solar heat may require metering, just like the other utilities.

Accurately measuring the heat flowing through a pipe requires continuous monitoring of two temperatures and the liquid flow. DT controllers have the capability of monitoring at least two temperatures, and the latest models include a flow sensor or a flow estimate capability to calculate Btu.

Flow metering. A flow meter coupled with temperature differential and two temperature gauges helps accurately measure the heat-transfer fluid’s flow rate, making it easy to calculate how much energy your system is producing.

  • Grundfos’s Vortex Flow Sensor is a simple and robust flow meter. It has no moving parts, using a Venturi restriction with a solid state sensor to convert fluid motion into a flow signal. When combined with the temperature “In” and the temperature “Out” of any hot water control sensor, the Btu heat delivery can be calculated.

Memory cards & communication ports. Some solar controllers include removable memory cards and communications, offering users the ability to store and download data to their computers for more detailed system tracking and performance analysis. The displays, while still rudimentary, are becoming more graphical, animated, and user-friendly.

Multifunction SHW controls. Buderus, Caleffi, ReSol, Steca, and Tekmar all offer multifunction controls for more complex solar heating systems. The complexity of electrical connections and sophisticated computer logic escalates quickly in larger home-heating applications.

  • ReSol’s DeltaSol BX includes up to three differential thermostats, an adjustable thermostat control, and the capability to automate overheating protection. It can control up to four circulator pumps, has two variable speed outputs, connections for two kinds of flow meters, Btu monitoring, data logging, and five temperature sensors. This unit can control 26 pre-defined heating system configurations, making it exceptionally versatile.
  • SolarLogic’s SLIC control takes a different approach to whole-home heating and combisystems, replacing all conventional controls with a single box with built-in software. All the pumps, sensors, thermostats and zone valves are wired directly to the single control. The SLIC can control solar and backup heat sources, 10 heating zones, domestic hot water, a heat storage tank, both a pool and spa, and an ice-melting system. It uses a vortex flow meter to calculate solar production, and monitors both glycol pressure and boiler water pressure. A pH meter monitors the glycol’s condition. Built-in data logging keeps tabs on the heating system continuously.

Design & Analysis Software

When comparing solar heating system scenarios, computer design software uses weather data to simulate the performance of a variety of solar heating equipment. Three popular programs are Polysun, T*Sol, and RETscreen. All will assist the user in determining design for both simple and complex solar water heaters including:  What is the monthly heating load? How much solar energy is available for collectors at a suitable tilt and orientation? How much solar heat is delivered and how much fuel is offset as a result? How do the costs and benefits compare to other design options?

  • Polysun (from Vela Solaris) and T*Sol (from Valentin Software) can provide a graphical representation of the solar heating system configurations. This can help the designer visualize the system and get a handle on the parts list. These two programs are available with various features, ranging from less than $200 for the most basic to more than $1,500 for the professional versions.
  • RETscreen was developed by Natural Resources Canada, and is free. It can evaluate energy production and savings, costs, emission reductions, and financial viability for various types of renewable energy and energy-efficient technologies (RETs). It is built into an Excel spreadsheet, and inputs vary with the project type. Solar collector and climate data for hundreds of locations in North America is included in the software. RETscreen does not provide the eye-popping graphics of the more expensive programs, but does provide some very useful analyses.
  • SolarLogic’s SLASH-D (SolarLogic Assisted Solar Heating Design) is solar design software that aids in designing a whole-house solar combisystem by using a standard piping configuration compatible with the SLIC control system. SLASH-D requires a minimum amount of information about the job, such as the size of the heated area, the number zones, and the size of the DHW hot water load.

The program then suggests the number of collectors and a recommended tilt and calculates monthly fuel savings. The program outputs piping diagrams of all primary and secondary plumbing and suggests a list of parts. Once the basic layout of the heating system has been determined, the output files from the SLASH-D become the input files for the SLIC controller so that it “knows” what components are in the system without further programming.


Bristol Stickney has worked on all facets of solar heating systems for more than 30 years. He holds a bachelor of science degree in mechanical engineering and is a licensed mechanical contractor in New Mexico. He holds several solar heating and control patents. He is the chief technical officer for SolarLogic in Santa Fe, where he develops solar heating control systems and design tools.

Boaz Soifer is a founder of SolarLogic and has been involved in contracting and distributing hundreds of solar heating systems since 1999. He is currently also the vice president of sales at Focused Energy, a national wholesale distributor of components for grid-tied PV systems.

Featured Products:

ArtTec • • Differential controller

Buderus • • Solar controls

Caleffi • • StarMax V drainback flat-plate collector; 225 series pump station; iSolar solar control

Dow Chemical Co. • • DowFrost PG

DuPont • • Bio-glycol PG

Dynalene • • Dynalene BioGlycol PG

Grundfos • • Vortex Flow Sensor

Heat Transfer Products • • SuperStor Contender solar storage tank with built-in heat exchanger; Phoenix Solar Water Heater

Heliodyne • • Gobi 404 square flat-plate collector

IMC Instruments • • Eagle 2 & D2 differential controllers

Oatey Co. • • CryoTek PG

PAW • • Pipe manifolds

ReSol • • DeltaSol AL differential controller; DeltaSol BX control; other solar controls

RETscreen • • RETscreen solar heating design software

SolarLogic • • SLIC solar control; SLASH-D solar heating design software

Solar Skies • • SS16 square flat-plate collector; Flexible Water Storage Tank

Steca • • TR0301U differential controller; other solar controls

Taco Inc. • • “00” series circulator pump; X-Pump pump station

Tekmar • • 156 differential controller; solar control

Triangle Tube • • Smart Multi Energy solar tank

Tyfo • • Tyfocor PG

Valentin Software • • T*Sol solar heating design software

Vaughn Manufacturing Corp. • • SEPCO SR solar storage tank with integrated heat exchanger

Vela Solaris • • Polysun solar heating design software

Viessmann • • Vitosol 100-F SH1B low-profile collector

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