cp = 4186;
kS = 1000;
Δ1 = T3 - Tin;
Δ2 = T4 - T2;
Pz = 1000;
m1 = 1;
m0 = 0.1;
m2 = 3;
Tin = 4;
T4 = 30;
Tw = 50;
ϵ = 
        0.5*((Tw + 273)/(Tw - Tout));
rce1 = 
        m0*Tin + k*m2*T4 == 
          m0*T2 + k*m2*T3;
 (*dostala voda*) rce2 = 
        m0*cp*(T2 - Tin) == 
          kS*((Δ1 + Δ2)/2);
 (*miseni*) 
rce3 = 
        k*m2*T3 + (1 - k)*m2*T4 == 
          m2*T5;
  
rce4 = 
        Pel + m2*cp*(T5 - Tout) == 
          m1*cp*(Tw - T6);
rce5 = 
        m1*cp*(Tw - T6) == ϵ*
            Pel;
rce6 = 
        cp*(m1 - m0)*(Tw - T7) == 
          Pz;
rce7 = (m1 - m0)*T7 + m0*T2 == 
          m1*T6;
rcedohr[k_] = {rce1, rce2, rce3, rce4, rce5, rce6, 
          rce7};
ClearAll[res1];
res1[k_] := Solve[rcedohr[k]][[1]]

Plot[ϵ /. res1[k], {k, 0, 1}, 
        PlotStyle -> {Hue[0.63], Thickness[0.008]}, 
        AxesLabel -> {"k[1]", "ϵ[k]"}];

Plot[((Pz + cp*m0*(Tw - Tin))/Pel) /. res1[k], {k, 0, 1}, 
      PlotStyle -> {Hue[0.63], Thickness[0.008]}, 
      AxesLabel -> {"k[1]", "ϵTotal[k]"}]