An Algorithm to Determine the Optimum Tilt Angle of a Solar Panel from Global Horizontal Solar Radiation

Table 2

Optimum tilt angle values and monthly mean of the daily global solar radiation values computed by means of the algorithm proposed in Section 2 applied to the monthly averages of the daily global horizontal solar irradiation data acquired from 1985 to 1989 at the site of Athens (latitude +37°58′N; longitude +23°43′E).

Solar declin. (deg)	Isotropic model		Le Quere’s model		Hay and Davies’ model		Klucher’s model		SEM values
	𝛽 (deg)	𝐸 (MJ/m^2)	𝛽 (deg)	𝐸 (MJ/m^2)	𝛽 (deg)	𝐸 (MJ/m^2)	𝛽 (deg)	𝐸 (MJ/m^2)	𝛽 (deg)	𝐸 (MJ/m^2)
𝑑1 = −21.269	50.1	6.58	60.5	7.42	53.5	6.73	60.2	9.89	2.22	0.66
𝑑2 = −13.289	42.3	9.42	52.7	10.66	45.8	9.68	52.2	12.16	2.19	0.54
𝑑3 = −2.819	33.0	13.16	42.4	14.94	36.2	13.59	40.9	14.65	1.87	0.37
𝑑4 = 9.415	24.7	20.47	30.2	22.59	27.1	21.24	28.8	20.13	1.03	0.47
𝑑5 = 18.792	17.1	24.75	20.5	26.65	18.7	25.56	18.9	23.23	0.61	0.62
𝑑6 = 23.314	13.6	28.03	15.8	29.58	14.8	28.82	14.6	26.03	0.39	0.66
𝑑7 = 21.517	15.7	29.27	17.7	30.82	16.9	30.15	16.8	26.97	0.36	0.73
𝑑8 = 13.784	22.9	27.03	25.9	28.83	24.7	28.03	25.3	25.33	0.55	0.65
𝑑9 = 2.217	33.6	21.73	37.8	23.49	36.0	22.62	38.2	22.39	0.90	0.31
𝑑10 = −9.599	43.2	14.21	49.5	15.64	46.2	14.77	50.6	17.48	1.45	0.62
𝑑11 = −19.148	48.2	7.38	58.5	8.32	51.6	7.56	58.2	10.63	2.19	0.65
𝑑12 = −23.335	52.3	5.92	62.5	6.67	55.6	6.06	62.2	9.35	2.18	0.69