Engineering Reference — EnergyPlus 9.0

<< Prev | Table of Contents | Next >>

Solar Collectors[LINK]

Solar collectors are devices that convert solar energy into thermal energy by raising the temperature of a circulating heat transfer fluid. The fluid can then be used to heat water for domestic hot water usage or space heating. Flat-plate solar collectors using water as the heat transfer fluid, Integral-Collector Storage solar collectors using water and unglazed transpired solar collectors using air are currently the only types of collector available in EnergyPlus.

Flat-Plate Solar Collectors[LINK]

The input object SolarCollector:FlatPlate:Water provides a model for flat-plate solar collectors that are the most common type of collector. Standards have been established by ASHRAE for the performance testing of these collectors (ASHRAE 1989; 1991) and the Solar Rating and Certification Corporation (SRCC) publishes a directory of commercially available collectors in North America (SRCC 2003).

The EnergyPlus model is based on the equations found in the ASHRAE standards and Duffie and Beckman (1991). This model applies to glazed and unglazed flat-plate collectors, as well as banks of tubular, i.e. evacuated tube, collectors.

Solar and Shading Calculations[LINK]

The solar collector object uses a standard EnergyPlus surface in order to take advantage of the detailed solar and shading calculations. Solar radiation incident on the surface includes beam and diffuse radiation, as well as radiation reflected from the ground and adjacent surfaces. Shading of the collector by other surfaces, such as nearby buildings or trees, is also taken into account. Likewise, the collector surface can shade other surfaces, for example, reducing the incident radiation on the roof beneath it.

Thermal Performance[LINK]

The thermal efficiency of a collector is defined as the ratio of the useful heat gain of the collector fluid versus the total incident solar radiation on the gross surface area of the collector.

η=(q/A)Isolar

where:

q is the useful heat gain

A is the gross area of the collector

Isolar is the total incident solar radiation.

Notice that the efficiency h is only defined for Isolar > 0.

An energy balance on a solar collector with double glazing shows relationships between the glazing properties, absorber plate properties, and environmental conditions.

qA=Isolarτg1τg2αabsT4absT4g2RradTabsTg2RconvTabsTairRcond

where:

τg1 is the transmittance of the first glazing layer

τg2 is the transmittance of the second glazing layer

αabs is the absorptance of the absorber plate

Rrad is the radiative resistance from absorber to inside glazing

Rconv is the convective resistance from absorber to inside glazing

Rcond is the conductive resistance from absorber to outdoor air through the insulation

Tabs is the temperature of the absorber plate

Tg2 is the temperature of the inside glazing

Tair is the temperature of the outdoor air.

The equation above can be approximated with a simpler formulation as:

qA=FR[Isolar(τα)UL(TinTair)]

where:

FR is an empirically determined correction factor

(τα) is the product of all transmittance and absorptance terms

UL is the overall heat loss coefficient combining radiation, convection, and conduction terms

Tin is the inlet temperature of the working fluid.

Substituting this into Equation [eq:SolarCollectorEta],

η=FR(τα)FRUL(TinTair)Isolar

A linear correlation can be constructed by treating FR(τα) and -FRUL as characteristic constants of the solar collector:

η=c0+c1(TinTair)Isolar

Similarly, a quadratic correlation can be constructed using the form:

η=c0+c1(TinTair)Isolar+c2(TinTair)2Isolar

Both first- and second-order efficiency equation coefficients are listed in the Directory of SRCC Certified Solar Collector Ratings.

Incident Angle Modifiers[LINK]

As with regular windows the transmittance of the collector glazing varies with the incidence angle of radiation. Usually the transmittance is highest when the incident radiation is normal to the glazing surface. Test conditions determine the efficiency coefficients for normal incidence. For off-normal angles, the transmittance of the glazing is modified by an incident angle modifier coefficient.

Kτα=(τα)(τα)n

Additional testing determines the incident angle modifier as a function of incident angle θ. This relationship can be fit to a first-order, linear correlation:

Kτα=1+b0(1cosθ1)

or a second-order, quadratic correlation:

Kτα=1+b0(1cosθ1)+b1(1cosθ1)2

The incident angle modifier coefficients b0 and b1 are usually negative, although some collectors have a positive value for b0. Both first- and second-order incident angle modifier equation coefficients are listed in the Directory of SRCC Certified Solar Collector Ratings.

The SRCC incident angle modifier equation coefficients are only valid for incident angles of 60 degrees or less. Because these curves can be valid yet behave poorly for angles greater than 60 degree, the EnergyPlus model cuts off collector gains for incident angles greater than 60 degrees.

For flat-plate collectors, the incident angle modifier is generally symmetrical. However, for tubular collectors the incident angle modifier is different depending on whether the incident angle is parallel or perpendicular to the tubes. These are called bi-axial modifiers. Some special flat-plate collectors may also exhibit this asymmetry. The current model cannot yet handle two sets of incident angle modifiers. In the meantime it is recommended that tubular collectors be approximated with caution using either the parallel or perpendicular correlation.

Incident angle modifiers are calculated separately for sun, sky, and ground radiation. The net incident angle modifier for all incident radiation is calculated by weighting each component by the corresponding modifier.

Kτα,net=IbeamKτα,beam+IskyKτα,sky+IgndKτα,gndIbeam+Isky+Ignd

For sky and ground radiation the incident angle is approximated using Brandemuehl and Beckman’s equations:

θsky=59.680.1388ϕ+0.001497ϕ2

θground=90.00.5788ϕ+0.002693ϕ2

where ϕ is the surface tilt in degrees.

The net incident angle modifier is then inserted into the useful heat gain Equation [eq:SolarCollectorqOverA697]:

qA=FR[IsolarKτα,net(τα)nUL(TinTair)]

Equation [eq:SolarCollectorEta698] is also modified accordingly.

η=FRKτα,net(τα)nFRUL(TinTair)Isolar

Outlet Temperature[LINK]

Outlet temperature is calculated using the useful heat gain q as determined by Equation [eq:SolarCollectorqOverA707], the inlet fluid temperature Tin, and the mass flow rate available from the plant simulation:

qA=˙mcp(ToutTin)

where:

˙m is the fluid mass flow rate through the collector

cp is the specific heat of the working fluid.

Solving for Tout,

Tout=Tin+q˙mcpA

If there is no flow through the collector, Tout is the stagnation temperature of the fluid. This is calculated by setting the left side of Equation [eq:SolarCollectorqOverA707] to zero and solving for Tin (which also equals Tout for the no flow case).

References[LINK]

ASHRAE. 1989. ASHRAE Standard 96-1980 (RA 89): Methods of Testing to Determine the Thermal Performance of Unglazed Flat-Plate Liquid-Type Solar Collectors. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 1991. ASHRAE Standard 93-1986 (RA 91): Methods of Testing to Determine the Thermal Performance of Solar Collectors. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Duffie, J. A., and Beckman, W. A. 1991. Solar Engineering of Thermal Processes, Second Edition. New York: Wiley-Interscience.

Solar Rating and Certification Corporation. 2004. Directory of SRCC Certified Solar Collector Ratings, OG 100. Cocoa, Florida: Solar Rating and Certification Corporation.

Integral-collector-storage (ICS) Solar Collector[LINK]

Solar collectors with integral storage unit models use SolarCollector:IntegralCollectorStorage object, and the characteristics parameter inputs of this collector are provided by the SolarCollectorPerformance:IntegralCollectorStorage object. This model is based on detailed Energy Balance equations of solar collectors that integrates storage in it. This model has two options to represent the collector bottom outside boundary conditions: AmbientAir, and OtherSideConditionsModel. AmbientAir simply applies outside air temperature using combined convection and radiation conductance, and the OtherSideConditionsModel applies combined radiation and convection models that exiats in a naturally ventilated cavity to represent the collector bottom outside boundary condition. The later boundary condition accounts for the shading of the collector on the underlying surface, hence, the ICS collector can be assumed as an integral part of the building envelope. Schematic diagram of a rectangular ICS solar collector is shown in Figure [fig:schematic-diagram-of-rectangular-integrated] below:

Schematic diagram of rectangular Integrated Collector Storage unit [fig:schematic-diagram-of-rectangular-integrated]

Solar and Shading Calculations[LINK]

The solar collector object uses a standard EnergyPlus surface in order to take advantage of the detailed solar and shading calculations. Solar radiation incident on the surface includes beam and diffuse radiation, as well as radiation reflected from the ground and adjacent surfaces. Shading of the collector by other surfaces, such as nearby buildings or trees, is also taken into account. Likewise, the collector surface shades the roof surface beneath it, hence no direct solar radiation incident on the roof surface. The collector and the roof outside boundary conditions should be specified as OtherSideConditionModel to account for solar collector shading impact on the roof surface.

Mathematical Model[LINK]

The integral-collector-storage (ICS) solar collector is represented using two transient energy balance equations shown below. These equations represent the energy balance equation for the absorber plate, and the water in the collector.

mpCpdTpdt=A[(τα)eIthpw(TpTw)Ut(TpTa)]

mwCwdTwdt=A[hpw(TpTw)Ub(TwTOSC)Us(TwTa)]˙mwCw(TwTwi)

where:

mpCp is the thermal capacity of the absorber surface, J/C

A is the collector gross area, m2

(τα)e is the transmittance-absorptance product of the absorber plate and cover system

It is the total solar irradiation (W/m2)

hpw is the convective heat transfer coefficient from absorber plate to water (W/m2-K)

ma is the overall heat loss coefficient from absorber to the ambient air (W/m2-K)

Tp is the absorber plate average temperature (C)

Tw is the collector water average temperature (C)

Ta is the ambient air temperature (C)

mwCpw is the thermal capacity of the water mass in the collector (J/C)

Us is the area-weighted conductance of the collector side insulation (W/m2-K)

Ub is the conductance of the collector bottom insulation (W/m2-K)

Tosc is the outside temperature of bottom insulation determined from the other side condition model (C)

Twi is the entering makeup or mains water temperature (C)

˙mwCw is the water capacity flow through the collector (W/C).

The other side condition model boundary condition represented by the Tosc, allows us to apply a realistic outside boundary condition for a collector mounted on a building roof. This also accounts for the shading impact of the collector on the under-laying surface (roof). On the other hand if ambient air boundary condition is specified, then the collector does not shade the underlying surface it is mounted on.

The two energy balance equation can be written as non-homogeneous first order DE with constant coefficients. The initial conditions for these equations are the absorber plate average temperature and the collector water average temperature at previous time steps.

dTpdt=a1Tp+a2Tw+a3

dTwdt=b1Tp+b2Tw+b3

a1=(Ahpw+AUt)/(mpCp)

a2=AhpwTw/(mpCp)

a3=A(τα)eIt+AUtTa

b1=AhpwTp/(mwCw)

b2=(Ahpw+AUb+AUs+˙mwCw)

b3=(AUbTosc+AUsTa+˙mwCwTwi)

The two coupled first order differential equation are solved analytically. Auxiliary equation of the the coupled homogeneous differential equation is given by:

λ2(a1+b2)+(a1b2a2b1)=0

This auxiliary quadratic equation has always two distinct real roots (λ1 and λ2) hence the solution of the homogeneous equation is exponential, and the general solutions of the differential equations are given by:

Tp=c1eλ1t+c2eλ2t+A

Tw=r1c1eλ1t+r2c2eλ2t+B

The constant terms A and B are the particular solution of the non-homogeneous differential equations, the coefficients of the exponential terms (c1, c2, r1, and r2) are determined from the initial conditions of the absorber and collector water temperatures (Tp0, Tw0) and are given by:

r1=(λ1a1)/a2;r2=(λ2a1)/a2

A=(a3b2+b3a2)/(a1b2b1a2);B=(a1b3+b1a3)/(a1b2b1a2)

c1=(r2Tp0Tw0r2A+B)/(r2r1);c2=(Tw0r1Tp0+r1AB)/(r2r1)

Thermal Network Model:[LINK]

The thermal network model requires energy balance for each of the collector covers as well. The heat balance equation of the collector covers is assumed to obey steady state formulation by ignoring their thermal mass. The thermal-network representation of the ICS collector is shown in Figure [fig:thermal-network-diagram-for-ics-solar]. Also, the heat balance at each cover surface requires knowledge of the amount of solar fraction absorbed, which is determined from the ray tracing analysis. For the thermal network model shown above the overall top heat loss coefficient is determined from combination of the resistances in series as follows:

Ut=[R1+R2+R3]1

or

Ut=[1hc,c1a+hr,c1a+1hc,c2c1+hr,c2c1+1hc,pc2+hr,pc2]1

The convection and radiation heat transfer coefficients in equation above are calculated based on temperatures at the previous time step and determined as described in the Heat Transfer Coefficients section.

Thermal network diagram for ICS Solar Collector [fig:thermal-network-diagram-for-ics-solar]

Collector Cover Heat Balance

Ignoring the thermal mass of the collector cover, a steady state heat balance equations are formulated for each cover that allow us to determine cover temperatures. The cover surface heat balance representation is shown in Figure [fig:collector-cover-surface-heat-balance] below.

Collector Cover Surface Heat Balance [fig:collector-cover-surface-heat-balance]

The steady state cover heat balance equation is given by:

qLWR,1+qCONV,1+qsolar,abs+qLWR,2+qCONV,2=0

Linearizing the longwave radiation exchange and representing the convection terms using the classical equation for Newton’s law of cooling, the equations for the temperatures of covers 1 and 2 are given by:

Tc1=αc1It+hr,c1aTa+hc,c1aTa+hr,c2c1Tc2+hc,c2c1Tc2hr,c1a+hc,c1a+hr,c2c1+hc,c2c1

Tc2=αc2It+hr,c2c1Tc1+hc,c2c1Tc1+hr,pc2Tp+hc,pc2Tphr,c2c1+hc,c2c1+hr,pc2+hc,pc2

where:

αc is the weighted average solar absorptance of covers 1 and 2

hr,c1a is the adjusted radiation heat transfer coefficient between cover 1 and the ambient air (W/m2-K)

hc,c1a is the convection heat transfer coefficient between cover 1 and the ambient (W/m2-K)

hr,c2c1 is the radiation heat transfer coefficient between covers 1 and 2 (W/m2-K)

hc,c2c1 is the convection heat transfer coefficient between covers 1 and 2 (W/m2-K)

hr,pc2 is the radiation heat transfer coefficient between covers 2 and the absorber plate (W/m2-K)

hc,pc2 is the convection heat transfer coefficient between covers 2 and the absorber plate (W/m2-K)

qLWR,1 is the longwave radiation exchange flux on side 1 of the collector cover (W/m2)

qCONV,1 is the convection heat flux on side 1 of the collector cover (W/m2)

qLWR,2 is the longwave radiation exchange flux on side 2 of the collector cover (W/m2)

qCONV,2 is the convection heat flux on side 2 of the collector cover (W/m2)

qsolar,abs is the net solar radiation absorbed by the collector cover (W/m2)

R is the thermal resistance for each section along the heat flow path (m2K/W).

Other Side Condition Model[LINK]

ICS Solar Collectors are commonly mounted on building heat transfer surfaces hence the collectors shade the underlying heat transfer surface and require a unique boundary condition that reflects the air cavity environment created between the bottom of the collector surface and the underlying surface. The other side condition model that allows us to estimate the other side temperature, Tosc, can be determined based on steady state heat balance using the known collector water temperature at the previous time step.

Illustration for Other Side Condition Model [fig:illustration-for-other-side-condition-model]

Ignoring thermal mass of the collector bottom insulation, steady state surface heat balance can be formulated on the outer plane of the collector bottom surface facing the cavity as shown in Figure [fig:illustration-for-other-side-condition-model]. The heat balance equation on the outer plane of the collector bottom surface is given by:

qcond+qconv,cav+qrad,cav=0

Substituting the equations for each term in the above equation yields:

UL(TwTosc)+hc,cav(Ta,cavTosc)+hr,cav(TsoTosc)=0

Simplifying yields the bottom insulation other side condition temperature:

Tosc=ULTw+hc,cavTa,cav+hr,cavTsoUL+hc,cav+hr,cav

The cavity air temperature is determined from cavity air heat balance as follows:

Ta,cav=hc,cavATosc+˙mventCpTa+hc,cavATsohc,cavA+˙mventCp+hc,cavA

where:

hr,cav is the linearized radiation coefficient for underlying surface in the cavity (W/m2-K)

hc,cav is the convection coefficient for underlying surface in the cavity (W/m2-K)

Tso is the outside face temperature of the underlying heat transfer surface (C)

˙mvent is the air mass flow rate due to natural ventilation (kg/s)

qcond is the conduction heat flux though the insulation and bottom (W/m2)

qconv,cav is the convection heat flux between the collector bottom outside surface and the cavity air (W/m2)

qrad,cav is the longwave radiation exchange flux between the collector bottom outside surface and the outside surface of the underlying surface (W/m2).

The cavity air temperature is determined from the cavity air energy balance. The air heat balance requires the ventilated cavity air natural ventilation rates. The calculation of the ventilation rate is described else where in this document. The SurfaceProperty:ExteriorNaturalVentedCavity, object is required to describe the surface properties, the characteristics of the cavity and opening for natural ventilation.

Heat Transfer Coefficients[LINK]

The equations used to determine for the various heat transfer coefficients in the absorber and water heat balance equations are given below. The absorbed solar energy is transferred to the water by convection. Assuming natural convection dominated heat transfer for a hot surface facing down and a clod surface facing down the following correlation for Nusselt number by Fujii and Imura (1972). The Nusselt number for hot surface facing down ward is given by:

Nu=0.56(GrPrcosθ)1/5105<GrPr<1011

The Nusselt number for hot surface upward and cold surface facing down is given by:

Nu=0.13(GrPr)1/3GrPr<5.0×108

Nu=0.16(GrPr)1/3GrPr>5.0×108

Gr=gβv(TpTw)L3c/gβv(TpTw)L3cν2ν2

Pr=ν/ναα

Tr=Tp0.25(TpTw)

hw=Nuk/NukLcLc

where:

θ is the angle of inclination of the collector to the vertical (radians)

g is the gravitation force constant, 9.806 (m/s2)

Qn is the reference properties where the thermo-physical properties are calculated (C)

Lc is the characteristic length for the absorber plate (m)

k is the thermal conductivity of water at reference temperature (W/m-K)

ν is the kinematic viscosity of water at reference temperature (m2/s)

α is the thermal diffusivity of water at reference temperature (m2/s)

βv is the volumetric expansion coefficient (1/K) evaluated at Tv, Tv = Tw+0.25(Tp-Tw)

Nu is the Nusselt number calculated for water properties at the reference temperature

Gr is the Grashof number calculated for water properties at the reference temperature

Pr is the Prandtl number calculated for water properties at the reference temperature.

The various radiation and convection heat transfer coefficients are given by the following equations. The convection heat transfer coefficients between the covers and the absorber plate are estimated from the empirical correlation for the Nusselt number for air gap between two parallel plates developed by Hollands et al. (1976) is:

Nua=1+1.44{11708(sin1.8β)1.6Racosβ}{11708Racosβ}+(Racosβ5830)1/1331

hc=Nuk/NukLL

hrpc2=σ(Tp+Tc2)(T2p+T2c2)1/εp+1/εc21

hrc1c2=σ(Tc1+Tc2)(T2c1+T2c2)1/εc1+1/εc21

The long wave radiation exchange coefficients between the outer collector cover and the sky and ground referencing the ambient air temperature for mathematical simplification are given.

hrc1s=Fsεc1σ(Tc1+Ts)(T2c1+T2s)(Tc1Ts)(Tc1Ta)

hrc1g=Fgεc1σ(Tc1+Tg)(T2c1+T2g)(Tc1Tg)(Tc1Ta)

hcc1a=hcc1s+hcc1g

The convection heat transfer coefficient from the outer cover to the surrounding air is given by:

hcc1a=2.8+3.0Vw

When the bottom surface boundary condition is AmbientAir, the combined conductance from the outer cover to the surrounding is calculated from the equation below (Duffie and Beckman, 1991).

hcomb=5.7+3.8Vw

The overall loss coefficient through the bottom and side of the collector-storage is estimated as follows:

Ub=ULb(Ab/A)

Us=[1ULs(As/A)+1hcomb]1

where:

εc1 is the thermal emissivity of collector cover 1

εc2 is the thermal emissivity of collector cover 2

Fs is the view factor from the collector to the sky

Fg is the view factor from the collector to the ground

Tc1 is the temperature of collector cover 1, (K)

Tc2 is the temperature of collector cover 2, (K)

Ts is the sky temperature, (K)

Tg is the ground temperature, (K)

k is the thermal conductivity of air (W/m K)

L is the air gap between the covers (m)

β is the inclination of the plates or covers to the horizontal (radians)

Vw is the wind speed (m/s)

ULb is the user specified bottom heat loss conductance (W/m2-K)

ULs is the user specified side heat loss conductance (W/m2-K)

Ab is the collector bottom heat transfer area (m2)

As is the collector side area (m2)

hcomb is the combined conductance from the outer cover to the ambient air (W/m2-K).

Transmittance-Absorptance Product

The transmittance-absorptance product of solar collector is determined using ray tracing method for any incident angle (Duffie and Beckman, 1991). This requires optical properties of the cover and absorber materials and the the transmittance-absorptance product for any incident angle is given by:

(τα)θ=τα1(1α)ρd

The transmittance of the cover system for single and two cover are given by:

τ=12[(τ1τ21ρ1ρ2)+(τ1τ21ρ1ρ2)]

ρ=12[(ρ1+τρ2τ1τ2)+(ρ1+τρ2τ1τ2)]

The effective transmittance, reflectance and absorptance of a single cover are given by:

τ=τa21r1+r[1r21(rτa)2]+1r1+r1r21(rτa)2

ρ=12[r+(1r)2τ2ar1(rτa)2]+r+(1r)2τ2ar1(rτa)2

α=(1τa)2{(1r1rτa)+(1r1rτa)}

The transmittance of the cover system with absorption only considered ta, is defined as:

τa=exp(KLcosθ2)

θ2=sin1(sinθ1n1n2)

The reflectance of un-polarized radiation on passing from medium 1 with reflective index n1 to medium 2 with reflective index n2 is given by:

r=sin2(θ2θ1)sin2(θ2+θ1)

r=tan2(θ2θ1)tan2(θ2+θ1)

The sky and ground reflected diffuse radiations average equivalent incident angles are approximated by Brandemuehl and Beckman correlation (Duffie and Beckman, 1991) as follows:

θsd=59.680.1388β+0.001497β2

θgd=900.5788β+0.002693β2

where:

τ is the transmittance of the cover system

τ1 is the transmittance of the cover 1

τ2 is the transmittance of the cover 2

δ is the absorptance of the absorber plate

ρd is the diffuse reflectance of the inner cover

L is the thickness of a cover material (m)

K is the extinction coefficient of a cover material (m1)

θ1 is the angle of incidence (degrees)

θ2 is the angle of refraction (degrees)

r is the parallel component of reflected un-polarized radiation

r is the perpendicular component of reflected un-polarized radiation

Sm is the slope of the collector (degrees)

θsd is the equivalent incident angle for sky diffuse solar radiation (degrees)

θgd is the equivalent incident angle for ground diffuse solar radiation (degrees).

The integral collector storage unit thermal performance parameters are calculated as follows:

Qdelivered=˙mwCw(TwTwi)

QStored=mwCwdTwdt

QSkinLoss=A[Ut(TpTw)+Ub(TwTOSC)+Us(TwTa)]

ηthermal=mwCwdTwdt+˙mwCw(TwTwi)AIt

References:[LINK]

Duffie, J.A., and W.A. Beckman. 1991. Solar Engineering of Thermal Processes, 2d ed. New York: John Wiley & Sons.

Kumar, R. and M.A. Rosen. Thermal performance of integrated collector storage solar water heater with corrugated absorber surface. Applied Thermal Engineering: 30 (2010) 1764–1768.

Fujii, T., and H. Imura. Natural convection heat transfer from aplate with arbitrary inclination. International Journal of Heat and Mass Transfer: 15(4), (1972), 755-764.

Photovoltaic Thermal Flat-Plate Solar Collectors[LINK]

Photovoltaic-Thermal solar collectors (PVT) combine solar electric cells and thermal working fluid to collect both electricity and heat. Athough there are currently comparatively few commercial products, PVT research has been conducted for the past 30 years and many different types of collectors have been studied. Zondag (2008) and Charalambous et. al (2007) provide reviews of the PVT literature. Because PVT is much less commercially-mature, there are no standards or rating systems such as for thermal-only, hot-water collectors. EnergyPlus currently has one simple model based on user-defined efficiencies but a more detailed model based on first-principles and a detailed, layer-by-layer description is under development.

The PVT models reuse the PV models for electrical production. These are described elsewhere in this document in the section on Photovoltaic Arrays-Simple Model

Simple PVT Thermal Model[LINK]

The input object SolarCollector:FlatPlate:PhotovoltaicThermal provides a simple PVT model that is provided for quick use during design or policy studies. The user simply provides values for a thermal efficiency and the incident solar heats the working fuild. The model also includes a cooling mode for air-based systems where a user-provided surface emmittance is used to model cooling of the working fluid to the night sky (water-based cooling will be made available once a chilled water storage tank is available). No other details of the PVT collector’s construction are required as input data.

The simple model can heat either air or liquid. If it heats air, then the PVT is part of HVAC air system loop with air nodes connected to an air system. If it heats liquid, then the PVT is part of plant loop with nodes connected to a plant loop and the plant operating scheme determines flows.

Air-system-based PVT modeling include a modulating bypass damper arrangement. Control logic decides if the air should bypass the collector to better meet setpoint. The model requires a drybulb temperature setpoint be placed on the outlet node. The model assume the collector is intended and available for heating when the incident solar is greater than 0.3 W/m2 and otherwise it is intended for cooling. The inlet temperature is compare to the setpoint on the outlet node to determine if cooling or heating are beneficial. If they are, then the PVT thermal models are applied to condition the air stream. If they are not beneficial, then the PVT is completely bypassed and the inlet node is passed directly to the outlet node to model a completely bypassed damper arrangement. A report variable is available for bypass damper status.

Plant-based PVT do not include a bypass (although one could be used in the plant loop). The collector requests its design flow rate but it otherwise relies on the larger plant loop for control.

When the PVT themal collector is controlled to be “on,” in heating mode, and working fluid is flowing, the model calculates the outlet temperature based on the inlet temperature and the collected heat using the following equations.

Qtherm=AsurffactivGTηthermal

where:

Qtherm is the thermal energy collected (W)

Asurf is the net area of the surface (m2)

factiv is the fraction of surface aire with active PV/T collector

ηthermal is the thermal conversion efficiency.

Tout=Tin+Qtherm˙mcp

where:

Tout is the temperature of the working fluid leaving the PV/T

Tin is the temperature of the working fluid entering the PV/T

˙m is the entire mass flow of the working fluid through the PV/T

cp is the specific heat of the working fluid.

For air-based systems, the value of Tout is then compared to the temperature setpoint on the outlet node. If Tout exceeds the desired outlet temperature, Tset,out, then a bypass fraction is calculated to model a modulating bypass damper using:

fbypass=(Tset,outTout)(TinTout)

When the PVT themal collector is controlled to be “on,” in cooling mode, and working fluid is flowing, the model calculates the outlet temperature based on the inlet temperature and the heat radiated and convected to the ambient using a heat balance on the outside face of the collector:

˙mcp(TinTout)=˙QLWR+˙Qconv

where:

˙QLWR is the net rate of long wavelength (thermal) radiation exchange with the air, night sky, and ground. See the section “External Longwave Radiation” in the Outside Surface Heat Balance, for full discussion of how this modeled in EnergyPlus using linearized radiation coefficients.

˙Qconv is the net rate of convective flux exchange with outdoor air. See the section “Exterior/External Convection” in the Outside Surface Heat Balance, for full discussion of how this modeled in EnergyPlus. The surface roughness is assumed to be “very smooth.”

The simple model assumes that the effective collector temperature, Tcol , is the average of the working fluid inlet and outlet temperatures so that we can make the following substitution:

Tout=2TcolTin

Substituting and solving for Tcol we obtain the following model for collector temperatures during a (possible) cooling process :

Tcol=2˙mcpTin+Asurffactiv(hr,gndTgnd+hr,skyTsky+hr,airTair+hc,extTair)2˙mcp+Asurffactiv(hr,gnd+hr,sky+hr,air+hc,ext)

Then the outlet temperature can be calculated and heat losses determined. However, the model allows only sensible cooling of the air stream and limits the outlet temperature to not go below the dewpoint temperature of the inlet.

PVT collectors have a design volume flow rate for the working fluid that is autosizable. For air-based systems used as pre-conditioners, the volume flow rate is sized to meet the maximum outdoor air flow rate. For water-based systems on the supply side of a plant loop, each of the PVT collectors are sized to the overall loop flow rate. For water-based systems on the demand side of a plant loop, the collectors are sized using a rule-of-thumb for typical flow rates per unit of collector area. This rule-of-thumb is based on a constant factor of 1.905x105 m3/s-m2 that was developed by analyzing SRCC data set for conventional solar collectors (see data set SolarCollectors.idf) and averaging the ratio for all 171 different collectors.

References[LINK]

Charalambous, P.G., Maidment, G.G., Kalagirou, S.A., and Yiakoumetti, K., Photovoltaic thermal (PV/T) collectors: A review. Applied Thermal Engineering 27 (2007) 275-286.

Zondag, H.A. 2008. Flat-plate PV-Thermal collectors and systems: A review. Renewable and Sustainable Energy Reviews 12 (2008) 891-959.

Unglazed Transpired Solar Collectors[LINK]

The input object SolarCollector:UnglazedTranspired provides a model for transpired collectors that are perhaps one of the most efficient ways to collect solar energy with demonstrated instantaneous efficiencies of over 90% and average efficiencies of over 70%. They are used for preheating outdoor air needed for ventilation and processes such as crop drying.

In EnergyPlus, an unglazed transpired solar collector (UTSC) is modeled as a special component attached to the outside face of a heat transfer surface that is also connected to the outdoor air path. A UTSC affects both the thermal envelope and the HVAC air system. From the air system’s point of view, a UTSC is heat exchanger and the modeling needs to determine how much the device raises the temperature of the outdoor air. From the thermal envelope’s point of view, the presence of the collector on the outside of the surface modifies the conditions experienced by the underlying heat transfer surfaces. EnergyPlus models building performance throughout the year and the UTSC will often be “off” in terms of forced airflow, but the collector is still present. When the UTSC is “on” there is suction airflow that is assumed to be uniform across the face. When the UTSC is “off” the collector acts as a radiation and convection baffle situated between the exterior environment and the outside face of the underlying heat transfer surface. We distinguish these two modes of operation as active or passive and model the UTSC component differently depending on which of these modes it is in.

Heat Exchanger Effectiveness[LINK]

The perforated absorber plate is treated as a heat exchanger and modeled using a traditional effectiveness formulation. The heat exchanger effectiveness, εHX , is determined from correlations derived from small-scale experiments. Two correlations available in the literature are implemented in EnergyPlus. The first is based on research by Kutscher at the National Renewable Energy Laboratory. The second is based on the research by Van Decker, Hollands, and Brunger at the University of Waterloo. Because both correlations are considered valid, the choice of which correlation to use is left to the user.

Kutscher Correlation[LINK]

Kutscher’s (1994) correlation encompasses surface convection between the collector and the incoming outdoor air stream that occurs on the front face, in the holes, and along the back face of the collector. The correlation uses a Reynolds number based on the hole diameter as a length scale and the mean velocity of air as it passes through the holes as the velocity scale:

ReD=VhDν

where:

Vh is the velocity through the holes (m/s)

D is the hole diameter (m)

ν is the kinematic viscosity of air (m2/s).

The correlation is a function of Reynolds number, hole geometry, the free stream air velocity, and velocity through the holes:

NuD=2.75[(PD)1.2Re0.43D+0.011σReD(UVh)0.48]

where:

P is the pitch, or distance between holes (m)

D is the diameter of the holes (m)

σ is the porosity, or area fraction of the holes

Vh is the mean velocity of air passing through the holes (m/s)

U is the free stream velocity or the local wind speed (m/s).

The Nusselt number is formulated as:

NuD=U Dk

where:

U is the overall heat transfer coefficient based on log mean temperature difference (W/m2-K)

k is the thermal conductivity of air (W/m-K).

The heat exchanger effectiveness is:

εHX=1e[U A˙m cp]

Kutscher’s relation was formulated for triangular hole layout, but based on Van Decker et al. (2001) we allow using the correlation for square hole layout and scale P by a factor of 1.6.

Van Decker, Hollands, and Brunger Correlation[LINK]

Van Decker et. al. extended Kutscher’s measurements to include a wider range of collector parameters including plate thickness, pitch, suction velocities, and square hole patterns. Their model formulation differs from Kutscher’s in that the model was built up from separate effectiveness models for the front, back, and holes of the collector. Their published correlation is:

εHX=[1(1+ResMax(1.733Re1/122w,0.02136)1)]×[1(1+0.2273Re1/122b)1]×e(0.01895PD20.62ReDtD)

where:

Res=VsPv

Rew=UPv

Reb=VhPv

Vs is the average suction velocity across the front face of the collector (m/s)

t is the collector plate thickness (m).

Heat Exchanger Leaving Temperature[LINK]

Using either of the correlations above allows determining the heat exchanger effectiveness from known values. By definition the heat exchanger effectiveness is also:

εHX=Ta,HXTambTs,collTamb

where:

Ta,HX is the temperature of the air leaving the collector and entering the plenum (C)

Ts,coll is the temperature of the collector’s absorber plate (C)

Tamb is the temperature of the ambient outdoor air (C).

By rewriting Equation [eq:SolarCollectorHXEffect716] to solve for Ta,HX, we see that the temperature of the heated outdoor air entering the plenum can be determined once the collector surface temperature is known:

Ta,HX=εHXTs,coll+(1εHX)Tamb

Collector Heat Balance[LINK]

The collector is assumed to be sufficiently thin and high-conductivity so that it can be modeled using a single temperature (for both sides and along its area). This temperature Ts,coll is determined by formulating a heat balance on a control volume that just encapsulates the collector surface. The heat balances are formulated separately for active and passive modes and are diagrammed in the following figure.

Observe that for the passive case, we do not use the heat exchanger relations to directly model the interaction of ventilating air with the collector. This is because these relations are considered to not apply when the UTSC is in passive mode. They were developed for uni-directional flow (rather than the balanced-in-and-out flow expected from natural forces) and for specific ranges of suction face velocity. Therefore, this heat transfer mechanism is handled using classical surface convection models (as if the collector was not perforated). (Air exchanges are modeled as ventilation in the plenum air heat balance but do not interact with the hole edges in the collector surface.)

Transpired Collector Heat Balance [fig:transpired-collector-heat-balance]

When the UTSC is active, the heat balance on the collector surface control volume is:

q′′αsol+q′′LWR,Env+q′′conv,windq′′HX+q′′LWR,plen+q′′source=0

where:

q′′αsol is absorbed direct and diffuse solar (short wavelength) radiation heat flux.

q′′LWR,Env is net long wavelength (thermal) radiation flux exchange with the air and surroundings.

q′′conv,wind is surface convection flux exchange with outdoor air under high wind and rain conditions. Note that this term is usually assumed to be zero in UTSC model development but we add the term to allow for deteriorated performance of the UTSC under poor conditions.

q′′HX is heat exchanger flux from collector to incoming outdoor air.

q′′LWR,plen is net long wavelength (thermal) radiation flux exchange with the outside face of the underlying surface(s).

q′′source is a source/sink term that accounts for energy exported out of the control volume when the collecter’s absorber plate is a hybrid device such as a photovoltaic panel.

While the heat balance on the passive collector surface control volume is:

q′′αsol+q′′LWR,Env+q′′conv,Env+q′′LWR,plen+q′′conv,