50,000 Square Foot Snowmelt Project

EzzyT

Thank you @Dan Foley for writing this great article.

https://www.phcppros.com/articles/20221-sim-system-atop-a-manhattan-car-dealers-roof-deck-parking-pad

EBEBRATT-Ed

What happens to the snow/ice on the cars? Sweep it on the deck and let it melt??

Nice job EzzyT

EzzyT

@EBEBRATT-Ed correct & thanks.

GGross

Those pex guns are great, have you used them before this project?

jumper

Wonder why not light weight water heater on roof?

EzzyT

@GGross yes those PexGun are well worth there weight in gold. This was the first project we used them on and have used them on many more projects since then.

@jumper light weight water heater am I missing something

jumper

" light weight water heater am I missing something"

Maybe? Boiler closer to terminal. CI no longer less expensive than copper tube. Low temperature.

EzzyT

@jumper the boilers used on this job are high efficiency condensing. When the boilers are on they are condensing like a water fountain.

GroundUp

Wow, well done Ezzy! Beautifully executed, as usual. I did not realize that Mr Pex was only 50 miles away from me, I drive through Ramsey on a near daily basis. Maybe they'll call me for the next one in Minneapolis :-P

CBRob

Wow, nice project.

Did you use a single tekmar 654 to control everything?

EzzyT

Thanks @GroundUp, very cool project to work on.

Thanks @CBRob, each mechanical has there own Tekmar 654 and are linked together with a Tekmar 486

Mark Eatherton

Great job, with an even GREATER potential for saving energy, if applicable. I'm assuming that with this being in downtown NYC, the air is dirty and the cars get dirty, requiring a LOT of car washing. I have an idea I call the "Why Not?" system. I will just leave this here, and if anyone has questions, I'd be glad to answer them as they come.

PART 1

A proposed guideline for the harvesting of free solar and ambient energy from the surface of a snowmelt slab.

Explanation of a typical SIM (Snow, Ice melt) hydronic system.

A conventionally fueled SIM system consists of a heat source (gas, LP, oil, solid fuels, solar thermal, waste heat etc.), an in slab tubing system, typically 5/8” dia. PEX tubing at 9” on center with a maximum length of less than 300 ‘, a minimum of 1” XPS insulation at the bottom and edges of the slab profile, distribution main piping, a boiler circulator and a system circulator, along with a means of controlling the operation of the SIM system. 

At present, the controls for running these systems looks at the following parameters to determine when it runs and when it turns off.

1. Moisture sensor (either aerial or in slab)
2. Slab sensor
3. Boiler supply sensor
4. Boiler return (if non condensing or solid fuel)
5. SIM System supply sensor
6. SIM System return sensor
7. OSA (out side air) temperature sensor (preferably on a North exposure)
8. Possibly an HID (Human Interface Device) consisting of a either a 12 hour twist timer, or a push button to begin a timer sequence within the control (Tekmar for example).

SEQUENCE OF OPERATION: Once the ambient temperature is down to a certain condition, (typically 35 to 40 OSA temperature), AND moisture is sensed at the moisture sensor, the heat source and distribution pumps are enabled and the typical target fluid temperature is around 140 degrees F. (This could be done on an OSA reset schedule, but currently isn’t by any manufacturer I am aware of. More ENV potential)

The SIM system will continue to run until until the target slab temperature is achieved. It will maintain this temperature until the moisture sensor is dry and a preprogrammed post run time factor (if used) has been achieved (tekmar incorporates this feature). 

An additional feature that may or not be incorporated into the control scheme is a condition called “Slab IDLEing”. 

A slab will have a tendency to drop below the ambient temperature due to a condition known as night sky re-radiation (NSR). It is not uncommon to see a slab temperature drop to 20 degrees F less than the actual surrounding ambient temperature during a cloudless night. On systems whose operation is critical (Hospital ramps, Helicopter pads, etc) the “IDLE” condition is used to keep the slab at a temperature which is above the NSR temperature, but below a MELT condition, which is typically around 40 degrees F. By keeping the slab IDLED, it reduces the transition time to bring the mass of the slab to a MELT (40 degrees F) condition. 

The SIM systems actual thermal load varies by location and weather variables, but the typical design for snow areas is around 150 Btu per square foot per hour. This is based on an ambient temperature of 10 degrees F, with a wind load of 10 MPH. This is the “worst case” scenario. The annual costs of operation depends on numerous variables including duration of snow events, number of snow events, wind loading, snow temperature and associated melt runoff. 

The solar radiation that falls upon a SIM slab can be harvested, even during the winter months, utilizing off shelf technology, known as a water source heat pump (WSHP)  coupled with a reverse indirect DHW heater, and a control logic to make it all come together (ENV). 

The additional control logic and points of sensing that would need to be added to make this possible are as follows:

End part 1

Mark Eatherton

Part 2

Sensor Points

1. Preheat Storage tank temperature (bottom 1/3)
2. OSA Relative humidity.
3. WSHP inlet temperature.

It has been known for many years, that the most efficient solar thermal solar collector is an unglazed plastic solar panel used for heating swimming pools (see for more information on unglazed low temperature applications.

The energy potential to be harvested off of a typical SIM slab exposed to the sun is enormous. The SIM is essentially the same as an unglazed solar panel. 

The energy harvested must be utilized in order for  the economics to work in a positive manner. The WSHP is capable of generating temperatures in the area of 115 degrees F. This energy could be used for low temperature radiant floor space heating, fan coil units, swimming pool heating, hot tub/spa heating. In addition to the harvestable solar gain, the slab is completely capable of harvesting “Ambient Energy”, that being the air temperature surrounding the slab during the warmer months. 

Concrete has a relatively low resistance to the transfer of heat, which means it is highly conductive and capable of allowing the WSHP to maintain a high co-efficient of performance (COP), typically 3:1 meaning it is 300% efficient at transferring heat. In fact the entering water temperatures of the WSHP will have to be”governed” per the manufacturers instructions to ensure that the entering heat transfer fluid (glycol HTF) temperatures are within the manufacturers maximum recommended limits. This will require either a 3 way mixing valve or a small variable speed injection pump on the inlet to the WSHP on the Source Side to ensure compliance. 

Sizing 

I’d recommend that the sizing of the WSHP be limited to 100 Btu’s per square foot of active surface area. This will dictate the size of the WSHP. For example, for each 5 tons of WSHP, I’d recommend a 120 gallon reverse indirect like this. 

Each 5 ton WSHP is capable of harvesting the solar energy off of approximately 600 square foot of SIM active panel, assuming full solar gain potential (317 Btu/sq.ft./hr at peak insolation, solar noon) to be harvested. 

As it pertains to the typical American DHW daily demands, this 5 ton system would equate to satisfying the needs of a 4 person family per hour. Larger storage medium equates to more harvested Btu’s, but also reduces the final temperature of operation of the preheat storage system. 

Bear in mind that with current refrigerants, 115 degrees F is the maximum recommended discharge temperature. The newer CO2 based refrigerants have an even higher discharge potential temperature, and are quickly coming to the market.

The “loads” that are connected to this system must be utilized as much as possible in order to guarantee seasonal efficiency. In other words, if the load is inadequate, there is virtually nowhere to send the harvested energy to. It would still produce FREE energy, but the life cycle economics would be negatively affected. It would be a novelty if the load is not there.

It is possible to incorporate the SIM systems output into a seasonal energy storage system (Thermal Battery) that also incorporates WSHP technology. Thermal Storage batteries are based on drawing the sensible heat out of water to create ice. Each degree temperature of difference in each pound of water equates to 1 Btu/pound, until the water goes through phase change at 32 degrees F where it changes to 144 Btu’s per pound when transitioning from 32 degree water to 32 degree ice. When transitioning back to a liquid form, it once again requires 144 Btu’s per pound to transition to 32 degree water, and then stays at 1 Btu/pound up to the other phase change which occurs at 212 degrees F. The SIM system with solar falling on it would retain a high COP for the WSHP transferring energy from the SIM system to the Thermal Battery.

Mechanical Control Points

The following control mechanisms must be considered in the ENV Control logic.

1. WSHP call for operation (Dry contact closure for Cooling mode of the slab)
2. 3 way Diverting Valve (SPST low voltage High Cv, low pressure drop)
3. Slab extraction circulator (can be a small VFD circulator with a 1 to 10 VDC control signal based on flow stream delta T of maintaining a 10 degree delta across the WSHP (P-1) or a simple contact closure for relay pilot voltage.
4. Constant speed circulator to transfer energy between the WSHP and the reverse indirect preheat storage tank (P-2). A simple contact closure for relay pilot voltage.
5. WSHP inlet governor using either a 3 way mixing valve or a small variable speed circulator WSHP over heat protection). This can be done via a SPDT relay controlling low voltage, or a 1 to10 VDC control signal controlling the pump based on a fixed set point.
6. OPTIONAL: WSHP Bypass Valve to allow for the direct input into the preheat storage tank whenever the slab temperature alone is capable of providing preheat without the use of the WSHP (slab temperatures in excess of 100 degrees F., which are typical during the Summer months. SPST low voltage to diverting valve.

Control Logic Parameters

Water Source Heat Pump operation: If the slab sensor is 20 degrees above the preheat storage tanks temperature, but slab is less than 90 degrees F. the WSHP, Diverting Valve, P1 and P2 are started. WSHP and associated pumps continue to run until;

1. Temperature differential between slab and storage tank are within 5 degrees F of each other, OR 
2. The slab temperature is within 5 degrees F of the sensed outside air dew point, which ever happens first, OR 
3. 40 degree F slab temperature (seasonally adjusted) is met. This parameter assumes the slab is still possibly being activated for a SIM event.

Water Source Heat Pump Bypass operation: If the slab sensor is 20 degrees above the preheat storage tanks temperature, and greater than 90 degrees F. the WSHP Bypass Valve, Diverting Valve, and P2 are started. WSHP Bypass Valve and associated pumps continue to run until;

1. Temperature differential between slab and storage tank are within 5 degrees F of each other, OR 
2. the slab temperature is within 5 degrees F of the sensed outside air dew point, which ever happens first.

DEW Point critical operation

It is entirely possible that the slab could be drawn low enough to bring it below the Dew Point. The slab would then condense the available moisture out of the ambient air. While this could prove to be beneficial to water plants along side the SIM slab (slope dependent) the pH of the condensate could prove to be problematic to the chemical makeup of the concrete, and would most probably support a bacterial growth (condensate slime) that should be avoided. By maintaining a 5 degree F buffer between the slab minimum and the variable dew point, this potential issue would be avoided. If a fast moving thunderstorm came though raising the Dew Point quickly, rain will most probably fall, causing the slab to become wet anyway. This scenario (condensation production) would most probably only occur where the thermal load exceeds the availability of solar and ambient energy. (i.e. 24 hour restaurant dish washing or other industrial hot water usage scenarios).

I will generate a mechanical drawing showing the mechanical and sensing points today to clarify the overall scenario.  In the mean time, feel free to share this with any potential customers that you may have. It is yours to use as you see fit.

So, "Why not?" Provided that the DHW physical plant is in close proximity to the SIM system, why not? It only makes sense.

ENjoy, and again, great job.

ME

Mark Eatherton

Mechanical schematic.

blob:https://forum.heatinghelp.com/42a30c50-dab4-41c0-90e9-8a16e84c20ac

hot_rod

the key to all of this is not unlike cogen units. You need a consistent thermal load to have it pencil out. They call these systems “thermally led” There have been several attempts to get cogen accepted both on the small residential systems, the Honda, and the FreeWatt, and the larger units that Lochinvar was promoting.

In all my years in the solar thermal business there were only a few applications where the large system numbers worked out. One was a water park in Wisconsin. It was open year around, indoor and outdoor pools, had a hotel, restaurant, convention center, & commercial laundry. So there was a pretty good chance that some thermal load was available every day.

Another smaller system was an ice rink in Michigan. Zambonis use hot water, so a system was sized to do several fills a day for the ice resurfacing. Hockey and figure skating is hard on the ice, so the Zamboni was always busy.

Where it could and should work is a districts system. Many customers to use the energy from that car lot, for example.