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					      WREF 2012: Early Results from a Solar Thermal Air Conditioning and Heating System

                                                        Eric Buchanan
                                                  University of Minnesota
                                          West Central Research and Outreach Center
                                                    46352 State Hwy. 329
                                                     Morris, MN 56267

ABSTRACT                                                         technologies, and obtain a LEED Silver Certification. One
                                                                 goal was to increase the building area by about 50%
The University of Minnesota, West Central Research and           without increasing net energy usage. A grant by the
Outreach Center (WCROC) commissioned a large                     university’s Institute of Renewable Energy and the
evacuated tube solar thermal system for building heating         Environment (IREE) made this possible by funding
and cooling (via an absorption chiller), along with a            renewable energy systems and energy efficient features for
Honeywell Building Management System for control and             the addition.
monitoring, in October, 2011.
                                                                 As part of this effort, an evacuated tube solar thermal
This paper will present early results from this system in        system and a Yazaki absorption chiller were installed. The
both heating and cooling mode, and discuss lessons learned       system is designed to provide air conditioning and space
during installation and operation of the system.                 heating for the new addition to the office building. The
                                                                 system was specified to provide 10 tons (35 kW) of cooling
Evacuated tube solar thermal panels and absorption chillers      and 15 tons (53kW) of heating.
are much less common in the United States than in Europe
and Asia, and such systems usually do not include the extra
sensors needed to thoroughly assess energy balances.             2. HVAC SYSTEM ARCHITECTURE
Performance data from this system will provide insight into
the effectiveness of such systems in a northern climate.         2.1 Primary system

                                                                 The original WCROC office building used ground source
1. INTRODUCTION                                                  heat pumps, with the ground loops configured in a
                                                                 horizontal trench, for heating and cooling. The remodel
The University of Minnesota West Central Research and            used this system, but replaced the heat pumps and added 10
Outreach Center (WCROC) is developing a field test               new vertical boreholes 200 ft. (61 m) deep. The final
facility for small scale renewable electric and thermal          system consists of 15 heat pumps ranging in size from 1 1/2
energy systems. The facility is a research, demonstration,       to 3 tons (5 to 10 kW). Heat energy is exchanged with air in
and certification platform that also allow for the practical     the occupied space via air ducts which route fresh and
use of the energy.                                               return air through the coils in the heat pumps. Fresh air is
                                                                 first brought in through energy recovery units where heat is
During the 2008 state legislative session, the WCROC             exchanged between the exiting and entering air.
received funding for a Renewable Energy Building
Addition. Construction was completed in the summer of            2.2 Solar thermal system
2010. This project was intended to be a model for
sustainable building design, demonstrate renewable energy

The system is designed to provide all the space heating and     The ground mounted solar panels are tilted at the latitude
cooling needs for the office building addition when the sun     angle (45) and face due south (fig. 1). They are configured
is shining. Otherwise, the space is conditioned by ground       in eight parallel branches of five series connected
source heat pumps. Various delays and redesigns prevented       collectors. The system operating mode is selected manually.
the solar energy system from being commissioned until
October of 2011.                                                2.2.2 Heating mode

2.2.1 Specifications and theory of operation                    In heating mode (Fig. 2), solar heated water is directed
                                                                through a mixing valve, which distributes it to the fan coils
Three heating/cooling coils were added to the air ducting       and/or the 300 gallon (1135 liters) storage tank. This valve
ahead of the heat pumps that serve the new building             can be controlled automatically so that when the load on the
addition. If solar heated or cooled water is available it is    heating coil is low, excess heat is stored in the storage tank.
sent through these coils. The heat pump fans run                When the load is high, more of the flow is directed to the
continually during the building’s occupied schedule,            coils.
providing air flow across the coils. If this is sufficient to
satisfy the room thermostats, the heat pumps will not be        The load on the coils is determined by measuring the
engaged and the space will be conditioned by solar energy       temperature drop across the coils. The control system
alone. If the thermostats are not satisfied the heat pumps      operates the mixing valve to maintain the temperature drop
will be engaged with the solar coils preconditioning the air    at a user defined set point. This has the effect of storing
prior to passing over the heat pump coils thereby reducing      heat when the coil load is low and mixing it into the loop
the load on them. The equipment used and key system             when the load is high. The storage tank can also be isolated
parameters are listed below.                                    at the end of the day, when it holds the maximum amount
                                                                of heat. The stored hot water can be circulated through the
   Conditioned space is 4700 ft2 (434 m2)                      coils the next morning to pre-heat the building.
   About 2100 ft2 (200 m2) of evacuated tube collector
    area (40 Solar Panels Plus, SPP-30 panels)
   Yazaki WFC-SC10 absorption chiller, water as
    refrigerant, lithium bromide as absorbent
   Heat transfer fluid is 50/50 mix of water and food
    grade propylene glycol
   System volume is 600 gallons (2270 liters) including
    300 gallons (1135 liters) of storage

The pump circulating fluid to the solar panels is operated by
a variable frequency drive (VFD) controller that varies the
pump speed to maintain a user selectable temperature set
point for the fluid in the collectors. The pump operates in
the range of about 10 gpm (38 l/min) to 50 gpm (189
l/min). Having a system that allows a flow rate below the
rated flow rate for a panel allows it to collect more useful
heat in low light conditions.

                                                                Fig. 2: Heating mode schematic

                                                                2.2.3 Cooling mode

                                                                In cooling mode (Fig. 3), solar heated water is directed to
                                                                the heat medium input of the Yazaki chiller and is then
                                                                returned to the solar panels. The chiller operates when the
                                                                heat medium input temperature is between 158F (70C)
                                                                and 203F (95C). In this mode, the fan coils are connected
Fig. 1: Evacuated tube solar thermal panels
                                                                to the output of the chiller forming a chilled water loop
                                                                separate from the solar panel hot water loop. The chilled

water loop includes the storage tank and a separate pump        added on a weekly basis. A solenoid opens a needle valve
switched on or off by the chiller.                              whenever the cooling tower pump is running to bleed high
                                                                mineral content water from the loop so it can be replaced
A mixing valve distributes chilled water to the coils or the    with fresh water.
storage tank; the same tank used in heating mode to store
hot water. The valve is controlled automatically by the
temperature of the water exiting the coils. When it is below
a user selected set point, load on the fan coils is low and
chilled water is stored in the storage tank. Otherwise,
chilled water in the tank is introduced into the loop when
the coil load is high.

                                                                Fig 4: Chiller             Fig 5: Cooling tower

                                                                2.2.4 Heat dissipation

                                                                Using a solar thermal system for space heating presents the
                                                                problem of what to do with the excess heat collected in the
                                                                summer. A heat dissipation method is needed for any
Fig 3: Cooling mode schematic                                   system designed primarily for space heating because there
                                                                will be times when the solar energy collected will exceed
                                                                the building thermal load. Having an air conditioning load Absorption chiller                                      provides a use for the large amount of summer thermal
                                                                energy collected, but doesn’t help in the spring or fall when
Using solar energy for cooling can provide an advantage in      there is no real need for heating or cooling.
system architecture because the peak cooling loads are
naturally aligned with the peak solar insolation. Air
conditioning can be produced from hot water via an
absorption chiller (Fig. 4). This works on the principle that
water will evaporate, in a vacuum, to be absorbed by a
liquid salt solution due to the attraction of the salt. The
evaporating water removes heat from a cooling circuit,
providing the air conditioning. Heat from the solar
collectors is used to liberate the absorbed water which is
then re-condensed to complete the cycle.

The heat removed during the absorption and condensation
processes is rejected to the environment via a wet cooling
tower (Fig. 5). A separate pump and plumbing loop is used
to move water between the cooling tower and chiller.
Chemical treatment to prevent scaling and bacteria growth
in the cooling tower water is also required. This can be
done with automatic chemical pumps, or, as in the system
described here, chemical tablets can be manually added to       Fig. 6: Heat dissipation
the cooling tower basin. When they dissolve, more are

Heat dissipation (Fig. 6) consists of standard slant fin       effective use. The sun rose at around 8 am, but chilled
tubing mounted to the back of the evacuated tube panels. A     water wasn’t available until almost noon. Of course, this
temperature controlled mixing valve diverts fluid through      situation should improve in the summer months. Early trial
the fin tubing before it enters the building when the          data with this system indicated it should be producing
temperature threshold is exceeded. The system attempts to      chilled water by about 10:30 am in mid-summer.
maintain the temperature threshold set point by varying the
proportion of the flow that is diverted.                       The overall efficiency number is still favorable compared to
                                                               a typical solar PV system that might be used to run an
                                                               electric air conditioner. An average PV panel has an
3.0 PERFORMANCE RESULTS                                        efficiency of about 14% converting sunlight into DC
                                                               electricity. This drops to about 11% after accounting for
The energy collected by the solar panels is calculated using   conversion to AC electricity and other de-rating factors.
the fluid temperature measurements going into and out of       This would be degraded further by the inefficiency of the
the panels and the measured flow rate. The fluid is a 50/50    air conditioner itself before the energy could be delivered to
mixture of propylene glycol and water and the data is          a building space.
averaged over six minute intervals. The energy delivered to
the building is calculated in the same way using the           Temperature measurements from the system are shown in
temperatures and flow rate across the HVAC coils.              Fig. 8 for Oct. 28th, and Oct. 29th, 2011. The outside air
Irradiation is measured by a pyranometer in the WCROC          temperature averaged 45F (7C) on both days during the
weather station.                                               time solar energy was collected.

Graphs of solar irradiation, the energy collected by the
panels, and the energy delivered to the office building are
shown below. The system was operated in cooling mode on
October 28th, 2011 (Fig. 7).

                                                               Fig.8: Solar system temperatures, Oct 28th & 29th, 201

                                                               The temperature of the fluid in the solar panels peaked at
                                                               about 180F (82C) which is below the chiller’s nominal
                                                               operating temperature of 190F (88C). The chiller
Fig. 7: Cooling mode energy balance, Oct 28, 2011              produced water that got down to 46F (8C) under these
                                                               conditions. These were not ideal cooling conditions, but it
                                                               was the first chance to measure the chiller’s performance.
The total insolation received per unit area on October 28th,
was 1049 Btu/ft2/day (3.32 kWh/m2/day). This corresponds       The coefficient of performance (COP) for a cooling device
to a cloudy day in the Solar Rating and Certification          is the ratio of the energy removed from the cooled space to
Corporation (SRCC) standard rating table. The amount of        the energy consumed by the device. This is a unit-less
insolation falling on the evacuated tube array (Fig. 7) was    number that indicates how many units of heat energy are
2173 kBtu (637 kWh). Of this, 37% was collected as heat        moved for each unit of input energy. The cooling device, in
by the solar panels and 11% was converted to chilled water     this case, includes the three system pumps in addition to the
and delivered to the building for cooling. Fig. 7 shows        chiller. The power consumption rates of the pumps were
graphically an often overlooked loss factor in solar thermal   measured and are listed in Table 1, along with the
systems; namely the amount of energy that is needed just to    consumption rate for the chiller. The solar pump consumed
get the system volume up to a useful temperature: 158F        less power than the other pumps because it is driven by a
(70C) for cooling. This also results in a delay before        VFD and was not running at full power. It will consume

about the same amount as the other pumps when it is            which just indicates the obvious benefit of using solar
running at full power in the summer.                           thermal energy to provide space heat.

TABLE 1: POWER CONSUMPTION RATES                               3.1 Heat storage mode

         Device                       Power (Watts)            Another operating possibility is to isolate the storage tank
       Solar Pump                          723                 when it has reached the highest temperature for the day, or
   Chilled Water Pump                     2666                 its upper storage limit of 180F (82C). This typically
   Cooling Tower Pump                     2038                 happens when the irradiation has fallen off to near zero in
          Chiller                          210                 the late afternoon. The remaining loop volume of 300
           Total                          5637                 gallons (1135 liters) continues to supply heat to the building
                                                               even after the building switches to unoccupied mode
                                                               (6:00pm). The next morning, the system can be switched to
Using the chiller energy output rate and the power             cooling mode so the hot water in the storage tank can be
consumption rates, the COP was found to vary between 1.8       circulated to the coils, and through the now passive chiller,
2.2 for the day. This seems promising considering the less     without going out to the solar panels. In this way stored
than optimal insolation.                                       heat can be used the next morning before the sun comes up.

The system was operated in heating mode on October 29th,       On December 10th, 2011, the total insolation received per
2011 (Fig. 9).                                                 unit area was 672 Btu/ft2/day (2.12 kWh/m2/day). This is a
                                                               little above the December average of 450 Btu/ft2/day (1.42
                                                               kWh/m2/day), but is still representative of the low
                                                               insolation during a Minnesota winter. The amount of
                                                               insolation falling on the evacuated tube array on the same
                                                               day (Fig. 10) was 1390 kBtu (407 kWh). Of this, 37% was
                                                               collected as heat by the solar panels and 34% was delivered
                                                               to the building for heating or stored for the next morning.

                                                               The outside air temperature averaged 30F (-1C) during
                                                               the time solar energy was collected. The maximum
                                                               temperature of the storage tank reached 172F (78C)
                                                               demonstrating the ability of the evacuated tube panels to
                                                               collect heat with low insolation levels. Moreover, no back-
                                                               up heat was required for the building addition after
                                                               11:30am until the next morning at 7:00am.
Fig. 9: Heating mode energy balance, Oct 29, 2011

The total insolation received per unit area on October 29th,
was 793 Btu/ft2/day (2.50 kWh/m2/day). This low level of
insolation is not addressed in the SRCC standard rating
table. The amount of insolation falling on the evacuated
tube array on that day (Fig. 9) was 1637 kBtu (480 kWh).
Of this, 31% was collected as heat by the solar panels and
29% was delivered to the building for heating. Fig. 9 shows
graphically that the amount of energy needed to get the
system volume up to a useful temperature is much less
since water heated to as little as 100F (38C) can provide
space heat. This also allows effective energy use earlier in
the day.
                                                               Fig. 10: Heating mode energy balance, Dec 10, 2011
The power consumption of the pump in heating mode was
only 441 W and more energy was delivered to the building
since solar collected heat is sent directly to the coils in    Fig. 10 graphically shows how the heat collected is
heating mode. This resulted in the COP peaking at 50           delivered over a greater portion of the day with the effective

use of heat storage. This is even more important for this        while good solar irradiation is present instead of starting
installation because the building is well insulated and has a    and stopping as the building thermostats dictate.
good southern exposure so little heat is actually needed on a
sunny afternoon when it is most available.                       An examination of fig.s 7, 9 and 10 demonstrates that the
                                                                 efficiency of a solar thermal system is interdependent on
For this system, the room temperature in the morning can         the system configuration and the thermal load placed on the
be consistently raised by 2F (1C) using heat in the storage    system, as well as, the solar energy available. Therefore,
tank. Moreover, the building needs very little heat at night     accurate thermal modeling of the entire system would be
because the thermostat set point drops to 62F (17C) at         very helpful in trying to design an efficient system. The
night from 68F (20C) during the day, and the room              system discussed here could certainly have benefited from a
temperature can be allowed to exceed the thermostat set          better analysis to optimize the array size and storage
point at the end of the day when solar heat is available. This   volume.
means the building addition is completely heated with solar
energy except for a few hours in the morning on sunny            Any thermal space conditioning system will produce excess
days.                                                            heat, at times, requiring a heat dissipation method. Overall
                                                                 system efficiency could be further enhanced by having a
The system is producing much more heat than the building         use for this “extra” heat. A swimming pool would make a
requires even on cloudy days, suggesting it was probably         good heat dump, but an even better solution would be a
oversized by about 50%. On the other hand, storage volume        long term storage method. There are products available that
is very small for a system of this size and a solar fraction     consist of super insulated large tanks that might be capable
approaching 100% could probably be attained with                 of storing heat from spring and fall to be used in the
sufficient heat storage capacity.                                summer and winter, respectively. As these are bound to be
                                                                 expensive, a good system model would be needed to assess
                                                                 the sizing and feasibility of such a system.

Configuring the system architecture to allow the storage         5.0 CONCLUSION
volume to be separated from the main solar loop and used
later, directly with the fan coils, offers several advantages.   Initial results from the solar thermal system installed at the
Namely, isolating the storage volume from the main loop in       West Central Research and Outreach Center (WCROC)
the morning allows the system to reach a useful operating        demonstrate the feasibility of using solar heat to provide air
temperature much sooner in the day. This is due to the           conditioning and the effectiveness of using it to provide
lower thermal inertia presented by the loop volume which is      space heating. It was also shown that the ability to separate
now smaller. This is true for both heating and cooling           the storage volume from the system can help make the
modes. Moreover, the chiller is allowed to operate over its      system more efficient, and allow the use of solar thermal
entire range of heat input temperatures.                         energy over a greater part of the day. Finally, it was
                                                                 determined that accurate thermal system modeling could
Typical installations using a water fired chiller employ a       greatly enhance the effectiveness of system design.
large buffer tank ahead of the chiller with an auxiliary heat
source to keep it at the nominal chiller heat input
temperature. More solar energy can be used in the                6.0 ACKNOWLEDGEMENTS
configuration described here because it becomes useful at a
lower temperature. Whether it is more efficient to use more      This work was made possible by a grant from the
solar energy and allow the chiller to operate under non-         University of Minnesota’s Initiative on Renewable Energy
optimal conditions, or to keep the chiller at optimal            and the Environment (IREE).
conditions with an auxiliary heat source is yet to be

Another advantage of this system configuration is that the
storage tank can be used to store hot or chilled water.
Storing chilled water is inherently more efficient because
the difference between the storage temperature and the
ambient temperature outside the tank is smaller. Storing
chilled water also allows the chiller to run continuously


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