Error in the 'for' function not storing values

I wrote this piece of code, and i can't understand what is wrong, keeps giving me this error:

*Attempted to access m(0); index must be a positive integer or logical.

Error in Solaretermico_codice (line 21) Ts(i) = (eff*Qs+eff*Qa+m(15-Ts(i-1)))/(Cp*M);*

Here it's the code:

%%Definizione Dati
n = size(orario,1); 
eff = 0.8;                  % Efficienza Serpentina
Cp = 4186;                  % Calore Specifico dell'acqua
m = portata;                % Consumo acqua calda in l/s
M = 500;                    % Capienza Serbatoio

%%Inizializzazione Variabili
Ts = zeros(n,1);
Ts(1) = 15; % in gradi °C

%%Inizio Programma
for i=2:n
    if Ts < 45;
        Qs = radiazione*4.9*0.7;
        Qa = 6000;
    elseif Ts < 55;
        Qs = radiazione*4.9*0.7;
        Qa = 0;
    end
    Ts(i) = (eff*Qs+eff*Qa+m*Cp(15-Ts(i-1)))/(Cp*M);
end

i'm sry if its in italian, i'll attach the file with the variables.

What i'm doing wrong? Also the Ts variable starts at 15 and then it creates 48 zeros, i'm missing something so that if saves the new Ts calculated at the end of the 'for' cycle?

Answer 1

[Star Strider]

The line that threw the error is not the same statement of the line that is in the code you posted. The current one is correct in that there is no subscript reference for m. Assuming no other errors, the code you posted should run.

[Giulio]

i changed the code into this one:

%%Definizione Dati
n = size(orario,1); 
eff = 0.8;                  % Efficienza Serpentina
M = 500;                    % Capienza Serbatoio

%%Inizializzazione Variabili
Ts = zeros(n,1);
Ts(1) = 15; % in gradi °C

%%Inizio Programma
for i=2:n
    if Ts < 45;
        Qs = radiazione*4.9*0.7 ;
        Qa = 6000 ;
        m = portata ;
        Cp = 4186 ;
    elseif Ts < 55;
        Qs = radiazione*4.9*0.7 ;
        Qa = 0;
        m = portata ;
        Cp = 4186 ;
    end
    Ts(i) = (eff*Qs+eff*Qa+m*Cp*(15-Ts(i-1)))/(Cp*M);
end

and now the error it gives it's this one:

*In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in Solaretermico_codice (line 23) Ts(i) = (eff*Qs+eff*Qa+m*Cp*(15-Ts(i-1)))/(Cp*M);*

[Star Strider]

As a temporary diagnostic step, just after:

%%Inizio Programma

insert:

n = 2

and change the Ts line to:

Ts = (eff*Qs+eff*Qa+m*Cp*(15-Ts(i-1)))/(Cp*M)

What is Ts? What size is Ts?

Also, you could have a problem once you get the Ts line sorted, since you’re creating Ts as a vector. The if and elseif lines may give unpredictable results. You might want to change those from Ts to Ts(i).

[Giulio]

i made the change u suggested, and it works. It's a simulation (extremly simplified) of a solar thermal energy plant.

Ts its the temperature of the water tank, and it should be in °C

the Ts vector that is created it's way too small, all the values are x10^(-3). So i'm doing something wrong on the scales?

[Star Strider]

I certainly encourage your solar energy research!

I suggest you check your constants. They may be off by a factor of 1E-3, so you may have one or more in the wrong units, perhaps kilocalories instead of calories, or some such. That’s usually the problem I discover in my calculations, especially when they’re off by some multiple of 1E±3.

[Giulio]

thank you for the encouragement! i changed Cp to 4.186 from 4186 and now i'm having decent results! but i'm seeing that the results are all in the form of 5.73 or a little bit higher... now it got me thinking that what i'm calculating is not the tank temperature, but the gain in °C made each 30 mins, that is why i'm finding such small values.

[Star Strider]

My pleasure!

It would seem to me that the gain in tank temperature is the important variable. That’s the energy capture and storage.