Solar Thermal Collectors
- 1 What is It?
- 2 Why is it Important?
- 3 When to Use It?
- 4 Green Garage Use of Solar Thermal Collectors
- 4.1 Sustainability Goals
- 4.2 Strategy and Conceptual Design
- 4.3 Integration and Controls Design
- 4.4 Supporting Science / Experience
- 4.5 Proposed Materials / Suppliers
- 4.6 Development Story
- 4.7 Related Internal Links
- 5 Resources
- 6 To Do's
What is It?
The solar thermal collectors subsystem captures the energy from the sun and transfers it to water as heat to provide space heating and domestic hot water for the building. It does so with one or more solar collectors positioned so the sun shines directly on them. The collectors are often located on top of buildings or in open areas adjacent to the building. The sun's energy heats up the collector. Water flowing through copper tubes in the collector is heated, and returned to mass thermal storage tanks. The stored hot water is then used for domestic hot water (e.g. showers) or space heating.
Solar thermal collectors are different from solar photovoltaic collectors in that they translate the sun's energy into heated water instead of electricity. A big difference is that solar thermal collectors are much less costly, but they are 3 to 4 times as efficient... an 80% efficiency versus 15% for photovoltaic. The efficiency of the solar thermal collectors varies greatly with delta T (Temp of the water in the collector -(minus) Temp outside air)...the greater the difference, the lower the efficiency of the collector.
There are two typical solar thermal panel types, flat panel and evacuated tube. They both have up to an 80% efficiency in converting the sun's energy into hot water. There's much debate as to which collector is best. It does appear that the flat panel has a longer life expectancy due to fewer points of failure. The tube seals on evacuated tubes are the weak link. Also, when looking at the efficiency of the collectors, you need to look at how efficient they are when placed into a system.
There are also two basic configuration designs for cold climates, closed loop glycol and open loop drainback. The main reason for the glycol is to prevent freezing of the water in cold winters, but a drainback design can also be used in cold climates.
A typical solar thermal panel subsystem includes:
- Solar collectors and framing to hold the collectors
- Piping carrying the water to and from the collectors and the thermal storage
- Thermal fluid...water and/or glycol
The size and number of collectors is determined by the amount of energy that is needed by the building, the location of the building, amount of sun energy available, and the efficiency of collectors.
- Also known as: Solar hot water collectors, Solar hot water, Solar hot water heating systems.
Why is it Important?
A solar thermal panel subsystem is important to a building's sustainability because it:
- Directly connects the building and its occupants to the earth's natural energy source: the sun. The sun is the source of all our energy.
- It is a renewable source of energy with no carbon footprint, except for the small pump (which may be run using photovoltaic collectors).
- Reduces the operating costs of the building by supplying the energy to heat the building and provide hot water.
When to Use It?
A solar thermal panel subsystem is best suited for locations where:
- The space heating demand has already been reduced through insulation of the the building envelope and domestic hot water needs have been reduced through use of low-flow shower heads and efficient appliances.
- There is adequate access to direct sun light in a location on or near the building for locating the solar thermal panel subsystem.
- There is an adequate demand for space heating and domestic hot water.
- You'd be looking for quickest payback renewable energy systems, as domestic hot water has one of the quickest paybacks of any renewable energy application.
Green Garage Use of Solar Thermal Collectors
The sustainability goals for the Solar Thermal Panel subsystem are:
- Meet the Green Garage space heating loads for the winter season per Energy-10 modeling results and our estimate for domestic hot water.
- 23 Million BTU per heating season (Nov15 - Mar 15) See GG Solar Thermal Workbook.
- Design for a 50 year life of the solar panel subsystem.
- Our heating energy usage would be only 10% of an equivalent commercial building (per ASHRAE data).
- Select the most efficient solar thermal panel system (= highest Heat Output / Total Life-cycle Cost).
- Connect the building and the occupants to the natural systems.
- Allow components of the system to be bypassed when they don't contribute to these goals.
- The system should be simple to maintain, adapt and control, and should position the Green Garage for a net-Zero energy future.
Strategy and Conceptual Design
Our overall strategy for the Solar Thermal Collectors is:
- Use the size of the flat roof of the side building as a limit to the number of collectors. We didn't want to deal with any structural issue associated with placing the collectors on the barrel roof of the historic building.
- Use thermal storage to reduce the number and size of the solar thermal collectors because we could store heat during, say, October when there is still a significant amount of sun, and and then use the heat in December when the amount of sun is minimal.
- Our collector selection strategy was to find the collector with the highest Heat Output / Total Life-cycle Cost: where Heat Output (Q-out) = temp rise x volume flow rate x specific heat of water. It should maximize BTUs generated per dollar invested. One additional less efficient panel may be a better choice than fewer, but more expensive, more efficient collectors. This is much less expensive than paying the high premium for the most efficient collector that is only slightly more efficient than the lower cost collector.
- Key Assumptions
- Flat Panel vs. Evacuated Tube: Flat panels are slightly lower cost and higher durability (twice the life)..performance usually better in the system vs. on the testing bench.
- Assumed the following collector energy production (per Build it Solar and Ranbow):
- Full Sun: 750 BTU/sf/day Winter
- Partly Sunny: 560 BTU/sf/day Winter
- Cloudy: 375 BTU/sf/day Winter
- Collector Type
- Use Flat Panels - because of longer life via simpler design.
- Collector Array Sizing
- Size = 400sf
- Each Collector = 4ft x 10ft (approx)
- Number of Collectors = 10
- Collector Weight = 150lbs panel + 10lbs water + 25lbs Framing + Hardware = 185lbs for each panel ... use 200lbs
- Collector Array Positioning
- On the flat roof of the side addition building. No sun blockage concerns (except for narrow chimney in early morning.)
- Vertical Angle: 60 - 70 degrees per SRCC guide + Alan Rushforth. The collectors should be (Latitude + 15 degrees) for winter heating driven systems
- Horizontal Angle: solar "true" south is 25 degrees west of a line perpendicular to the south wall. It is typically determine at site...5 - 8 degrees west of magnetic south. It is acceptable to have the horizontal alignment +/- 15 degrees off solar south. Sites for determining solar south at Build it Solar and determining a sunrise/sunset custom calendar
- Drainback Method
- Use a drainback design because of its simplicity. It eliminates the use of glycol, which is toxic and is the main cause of the deterioration of the collectors. It also eliminates the need for costly heat exchangers as the water is circulated directly into the thermal storage.
- The flow through the collectors would be triggered by a thermostat positioned inside the top of the collector reaching 90F (and above the temperature of the water in the tank.) Then the pump would begin pumping the water from the thermal storage tank up into and through the collectors. When the temperature dropped below 90F the pumps would stop. There is concern about the thermostat not operating correctly and water freezing in the panel. There should be a safety mechanism to ensure this doesn't happen (i.e. redundant thermostats ..both needing to be above 90F.)
- Collectors should be sloped at a 2% grade towards the pump to facilitate draining.
- Collector Array Configuration
- Connect the panel headers/footers in series and have the water flow through the collectors in parallel - this minimizes the amount of tubing between the collectors on the roof. Mounting the collectors in one straight row and connecting them with high temp silicone heater hose (clamped on, no solder joints or unions) keeps things more simple and efficient.
- Low pressure - with a drainback system, fluid in the lines is at atmospheric pressure +/-. With glycol, pressures are probably 15 to 25 psi.
- Flow Rate - when you are dealing with a drainback with no heat exchanger on the solar loop side, I feel 5 gpm/collector is adequate. I feel a full 1 gpm is wasting watts and adding wear and tear on the copper collector piping.
- Delta T - you will probably find your T in vs. T out is under 15 especially if you have more than about .6 gpm/collector.
- Collectors are butted up against one another with very short, high temp silicone hoses (1-1/8") connecting the headers/footers on the collectors.
- Collectors can be installed either portrait or landscape.
- Use insulated pex-al-pex to run/return between the collectors and the storage.
- Use differential controller - Stucca (located at the storage tank) w/ $10 sensors to determine when to send the water through the collectors. Place the sensors in the space at the top of the collector. Have it go on when it is > 95F.
- Overheating / Heat Dump Design
- With drainback approach, you have a little more leeway to handle occasional stagnations (no glycol to turn acidic), but some overheat protection is recommended.
- Option 1 - cover the panels
- 1A. Would like to see someone use the greenhouse tarp screens to rig something that could be manually controlled or controlled automatically from the ground.
- 1B. Another idea is to cover the solar panels with flexible, thin film PV material in the summer, April - September, to run cooling fans and pumps.
- Option 2 - use a heat dump...a radiator or sauna. You can't just leave the panels empty (drained) as the high heat build up would damage the panel.
- Remaining Design Issues
- How to we discourage damage...thrown rocks...etc.
Integration and Controls Design
Integrating the Solar Thermal Collector subsystem with the remaining parts of the geo-solar hybrid heating and cooling system does require some significant design effort. Some of the controls will be manual and some will be automated. The key integration and control areas are:
- The integration of the solar thermal collector array with the mass thermal storage tank system:
- in the heating mode for heating the water in the tanks by circulating the water through the panels on sunny days, and
- in the cooling mode used to cool the water in the tanks by circulate water on cool nights (i.e. a heat dump.)
- Connecting the solar thermal collectors to the in-ground pex at the perimeter of the annex building to store heat in the ground in the fall and winter. Thereby reducing the heat loss through the floor in the annex building.
- The drainback system of the solar panel system is temperature controlled.
- Covering (uncovering) most of the exposed solar panels in the summer to prevent overheating. This is expected to be done manually.
- Need controls to determine that there is a leak.
Key Design Criteria
Heating (Winter) Modes
- Occupied - Closed
- In heating mode, the solar panels are manually uncovered.
- The mass thermal storage tanks are automatically valved open to the solar thermal collectors subsystem.
- When the collector temperature sensor T-1A is above its minimum setpoint (90 deg F, adj) the solar collector system has the potential to generate heat and triggers an automatic test to see if the heat can be used. Below minimum setpoint, the system is not allowed to run.
- Mass Thermal Storage Tank Heating (Priority 1)
- The differential temperature between collector temperature T-1A and the mass thermal storage water temperature sensor T-2 is monitored by the system differential controller.
- When this differential temperature is greater than or equal to the start-up temperature differential setpoint (8 deg F, adj.), pump P-9 is activated, V-1 set closed, V-2 set open to tank, and V-4 set open to collectors (closed to trickle loop).
- Unheated water from the mass thermal storage tanks enters the solar panels, is heated, and returns to the mass thermal storage tanks.
- When the differential temperature between the mass thermal storage water temperature sensor T-2 and the solar thermal collector output temperature T-1B is less than the stop temperature differential setpoint (4 deg F, adj.), pump P-9 is deactivated, V-1 set open, V-2 set open to tank, and V-4 set open to collectors (closed to trickle loop) and the system drains by gravity back to the mass thermal storage tanks.
- Collector temperatures T-1A(in) and T-1B(out) and storage tank temperature T-2 are monitored by the controller for system performance as well as control, and solar heat output and pump run time are indicated by the control system.
- Domestic Hot Water Heating (Priority 2)
- Initially the solar thermal collectors will not be connected to the Altherma domestic hot water. The reason is the cost and complexity exceeds the benefits especially in the summer when the drainback into the cool tanks is counter productive.
- In-ground Annex Loop Heating (Priority 3)
- When the month is October through April (or manually indicated) this operation is available.
- When the the collector temperature sensor T-1A is above its minimum setpoint and the heat can not be used by higher priority heating needs, pump P-9 is activated, V-1 set closed, V-2 set closed, and V-4 set open to trickle loop (closed to collectors) allowing the heated water output from the solar collectors to be circulated through the annex in-ground loop (not radiant floor zone.)
- When the differential temperature between T-1C and T-1B is less than the stop temperature differential setpoint (4 deg F, adj.), pump P-9 is deactivated, V-1 set open, V-2 set open to tank, and V-4 set open to collectors (closed to trickle loop) and the solar collector system drains by gravity back to the mass thermal storage tanks.
- The in-ground loop valves will close and valves to the higher priority heating use will open while the pump P-9 continues, when a higher priority heating use calls for heat.
- Occupied - Open
- Same as Occupied - Closed hours heating mode.
- Same as Occupied – Closed hours heating mode.
- Mass Storage Tank Temperature (T-2) too high - If the storage tank temperature T-2 reaches its high temperature limit (200 deg F, adj.), an alarm is generated in the control system and V-4 is closed to the solar panels, so it cannot operate until the tank temperature is 10 deg F (adj.) below the tank heat limit (180 deg F, adj.).
- Mass Storage Tank Temperature (T-2) too low - If the storage tank temperature T-2 reaches its low temperature limit (80 deg F, adj.) and the Altherma has not begun to recharge the system, an alarm is generated in the control system and pump P-9 is deactivated until the alarm is manually reset.
- Solar Collector Outlet temperature (T-1B) too low (freeze protection) – If solar collector outlet water temperature T- 1B reaches 85 deg F (adj.) while operating, a warning is generated in the control system. If it reaches 80 deg F while the system is operating, an alarm is generated and pump P-9 is deactivated, V-1 set closed, V-2 set open to tank, and V-4 set open to collectors (closed to trickle loop) and the system drains by gravity back to the mass thermal storage tanks until the alarm is manually reset.
- Pump failure – If pump P-9 is called to run but does not start as monitored by the controller, an alarm is generated.
- ?Leak detection? – When system is in operation with pump P-9 running, system pressure is monitored at pressure sensor P-9?. If the pressure drops 3 psi (adj.) below the normal system operating pressure (determined during system test and balancing), an alarm is generated indicating possible leak in system. Pressure is not alarmed when system is off to prevent nuisance alarms.
Cooling (Summer) Modes
- Occupied - Closed
- In cooling mode, all solar panels are manually covered.
- The mass thermal storage tanks are automatically valved open to the solar thermal collectors.
- With system on, full flow is maintained through all solar panels, both covered and uncovered.
- Mass Thermal Storage Tank Cooling (Priority 1)
- When the system is manually set for "cooling mode" and the collector temperature sensor T-1A is below mass thermal storage water temperature sensor T-2 the solar collector system has the potential to "dump heat" into the air. This is expected during night / off-peak hours.
- When the differential temperature between T-1A and T-2 is greater than or equal to the cooling start-up temperature differential setpoint (6 deg F, adj.), pump P-9 is activated, V-1 set closed, V-2 set open to tank, and V-4 set open to collectors (closed to trickle loop.)
- Heated water from the mass thermal storage tanks enters the solar panels, is cooled, and returns to the mass thermal storage tanks.
- When the differential temperature between T-1B and T-1C is less than the stop temperature differential setpoint (4 deg F, adj.), pump P-9 is deactivated, V-1 set open, V-2 set open to tank, and V-4 set open to collectors (closed to trickle loop) and the system drains by gravity back to the mass thermal storage tanks.
- Collector temperature T-1A, T-1B and T-1C and storage tank temperature T-2 are monitored by the controller for system performance as well as control, and solar heat loss (absorption) and pump run time are indicated by the control system.
- Occupied - Open
- Same as Occupied - Closed hours cooling mode.
- Same during Occupied – Closed hours cooling mode.
- If outside air temperature is cooler than the cooling mass storage tank temperature T-2, the solar thermal collector system is allowed to operate during off peak hours as a heat exchanger to cool the water with the ambient air using the solar collectors as heat exchangers with the air. (See setpoints above.)
- Same as Emergency heating mode.
Shoulder (Spring - Fall) Modes
- Occupied - Closed
- The system is manually set to either heating or cooling mode.
- When the system is manually changed from:
- When changing from Cooling Mode to Fall Shoulder Mode, in preparation for entering Heating Mode the solar panels, are manually uncovered and remain so through the Heating Mode. It may be preferred to stage the uncovering, by gradually uncovering more collectors as the weather gets cooler.
- When changing from Heating Mode to Spring Shoulder mode in preparation for entering Cooling Mode the solar panels are manually covered and remain so through the Cooling Mode. It may be preferred to stage the covering, by gradually covering more collectors as the weather gets warmer.
- The system follows the sequence for whichever mode is active.
- Occupied - Open
- Same as Occupied - Closed hours shoulder mode.
- Same as Occupied – Closed hours shoulder mode.
- Same as Emergency heating mode.
Controls - Open Design Points
- Discuss mass thermal storage tank temperature sensor location since there are two tanks and the tanks may be used for different functions at different times. In shoulder season, when one tank is heating and one tank cooling, will the tanks always be designated for heating and cooling the same way (not interchangeable)? If so, then temperature sensor for solar heating could be in that designated tank and wired to the differential controller.
- Review emergency modes and confirm they are appropriate for intended system application and operation.
- Could low temperature/freeze protection backup sensor be located in domestic water tank near bottom and function as control in cooling mode as well as being freeze protection? That could save the cost of one DDC sensor. How could this backup sensor integrate with the packaged differential controller?
- Determine best location for system pressure sensor for leak detection (near end of pumping path?). Will pipe routing allow it to be in the path even when solar is doing domestic water only (cooling modes) with mass storage bypassed? Will pressure setpoint be different for heating vs. cooling mode (storage tanks out of the picture)?
- Controller literature mentions a “pump exercise” function that runs the pump 1 minute every 14 days. Is this included in our design or should it be?
- Check if pressure relief valves are required and have required setting per local code. Check temperature and pressure relief on domestic water.
- Provide piping and valves to isolate, bypass, and drain the solar collector system and the storage tanks. (manual control)
- Should air vents be provided at high point and at tank to help eliminate air when system is activating? Will vacuum relief vents also assist in drainback? 2008 ASHRAE HVAC System and Equipment says these may be undesirable because nonvented drainback loops do not require makeup water and eliminate corrosive effects of bringing air into the system. Confirm with design expert.
Supporting Science / Experience
There was a tremendous amount of research that went into this design. Our primary sources for information were:
- Alan Rushforth from Rushforth Solar LLC. He has extensive experience in developing Solar Hot Water systems for apartments buildings, many that he owns. A summary of our conversations with Alan can be found on our Conversation with Alan Rushforth on April 2, 2008 and Conversation with Alan Rushforth on January 13, 2011 pages. His designs are simply elegant.
- Bob Ramlow book ... Solar Water Heating
- Bob at SunEarth helped us get started in understanding how the solar collectors work. He's very helpful. A summary of our conversation with Bob can be found on our Conversation with Bob at SunEarth on March 23, 2008 page.
- Build it Solar website. Chris' excellent site on everything solar.
- SRCC site for certifying the solar collectors.
Proposed Materials / Suppliers
- Collector Manufacturers
- Alan Rushforth is now shifting from SolarHot collectors to Solene Cromagen - slightly better numbers and equally good pricing - just under $800 for a 4x10. His local distributor, Hickory Ridge Radiant, could probably help if there is no one in this area.
- Sun Earth
- Others: Solar Hot Water collectors highly recommended,
- AET (STSS recommended)
- Solar Thermal System
- Heliodyne large market share.
- Apricus ... the evacuated version that Roman used.
- Pex Manufacturers
- Use insulated pex-al-pex to run/return between the panels and the storage.
- Controllers and Pumps
- Use differential controller - Stucca
The Solar Thermal Collector subsystem - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our Solar Thermal Collector subsystem.
Related Internal Links
- Mass Thermal Storage
- Geothermal System
- Radiant Floor
- Hybrid Ventilation System
- Earth Tubes
- Natural Ventilation
- Moisture Control
- Energy Modeling
- Human Comfort Zone
- Super Insulated Building Envelope
- Bob Ramlow book ... Solar Water Heating is a great resource if you are starting out. Very clear, down to earth explanation of how to build a solar water heating system.
- Build it Solar website. Gary's excellent site on everything solar for the DIY-ers.
- SRCC - Solar Rating and Certification Corporation- Organization that certifies the performance of solar collectors in stand alone mode (OG100) and in a system (OG300). Here's their Collector Ratings page.
- Solar Thermal System Calculator Surpisingly sophisticated
- google data...check link to solar thermal data page...23 million for winter season.
- Short Video
- Upload images onto Development Story page
- Image for top of page?
- email Alan
- In Collector Array Positioning, sunrise/sunset data link not working
Joe was here
Peggy edited this page :)