lib_snowmodel.cpp

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#include "lib_snowmodel.h"
#include "lib_util.h"
#include <cmath>
#include <iostream>


#ifndef M_PI
#define M_PI 3.14159265358979323846264338327
#endif

/**********************************************************************************
************************************************************************************
**
**	6 April 2015
**
**	Implementation of Bill Marion's snow model [1] to C++ for use in SAM
**	Original author: David Severin Ryberg
**
**
**********************************************************************************
**  References:
**  1: Marion, Bill, et al. "Measured and modeled photovoltaic system
**     energy losses from snow for Colorado and Wisconsin locations."
**     Solar Energy 97 (2013): 112-121.
**
**********************************************************************************
***********************************************************************************/


pvsnowmodel::pvsnowmodel()
{
	mSlope = -80;
	//sSlope = (float)1.97;
	deltaThreshold = 1.00;
	depthThreshold = 1.00;
	previousDepth = 0;
	badValues = 0;
	maxBadValues = 500;
	coverage = 0;
	pCvg = 0;

	good = true;
	msg = "";

}

bool pvsnowmodel::setup(int nmody_in, float baseTilt_in, float snow_slide_coeff, bool limitTilt){

	nmody = nmody_in;
	baseTilt = baseTilt_in;
    sSlope = snow_slide_coeff;
	if(limitTilt && (baseTilt>45 || baseTilt < 10)){
		good = true;
		msg = util::format("The snow model is designed to work for PV arrays with a tilt angle between 10 and 45 degrees, but will generate results for tilt angles outside this range. The system you are modeling includes a subarray tilt angle of %f degrees.", baseTilt);
		return false;
	}

	good = true;
	return true;
}


bool pvsnowmodel::getLoss(float poa, float tilt, float , float tdry, float snowDepth, int sunup, float dt, float &returnLoss){

	bool isGood = true;

	// Check if snow depth value is valid
	// * 610 cm ~= 20 ft
	if (snowDepth < 0 || snowDepth > 610 || std::isnan(snowDepth)){
		isGood = false;
		snowDepth = 0;
		badValues++;
		if (badValues == maxBadValues){
			good = false;
			msg = util::format("The weather file contains no snow depth data or the data is not valid. Found (%d) bad snow depth values.", maxBadValues);
			return false;
		} 
	}
		
	/////////////////////////////
	// Step 1
	// look for snow fall and set current snow coverage amount accordingly
	if ((snowDepth - previousDepth) >= deltaThreshold*dt && snowDepth >= depthThreshold){
		coverage = 1;
	}
	else{
		coverage = pCvg;
	}

	// Coverage Override #1:
	//  This override assumes that if the current snow depth is less than the threshold,
	// then the snow coverage on on the PV arrays should be set to zero (even if the model
	// says otherwise).
	if (snowDepth < depthThreshold) coverage = 0;

	/////////////////////////////
	// Step 2
	//
	// Calculating Energy output is done outside this function

	/////////////////////////////
	// Step 3
	//
	// This model is adapted for hourly snow data, and so is not analyzed with
	// with respect to individual days, but instead for all 8760 yearly hours.
	// Hence, this step is skipped

	/////////////////////////////
	// Step 4
	//
	
	// If the day-time flag is not set, assume the tilt value should be equal to the system's base tilt
	//   This will fix the issue in sam where system tilt values during nightime hours is zero (even 
	//   for static systems). This may need to be altered for 1-axis tracking and 2-axis tracking systems
	if (sunup == 0){
		tilt = baseTilt;
	}

	// check if conditions are right for sliding
	if (tdry - poa / mSlope > 0){
		coverage -= (float)(0.1 * sSlope * sin(tilt * M_PI / 180) * dt);
	}

	// Coverage Override #2
	//  This override prevents the snow coverage from going below 0
	if (coverage < 0) coverage = 0;

	returnLoss = 0;
	if (nmody > 0) {
		returnLoss = ((float)ceil(coverage*nmody)) / nmody;
	}
	
	//  Current snow depth and previous snow depth are set to the correct values once
	// this function is run again

	previousDepth = snowDepth;
	pCvg = coverage;

	if (isGood) return true;
	else return false;
}