def calc_chiller_quality_grade(nominal_conditions):
    """
    Calculates the quality grade for a given point of operation.

    Note
    ----
    This function is rather experimental.
    Please do not use it to estimate the quality grade of a real machine.
    A single point of operation might not be representative!

    .. calc_chiller_quality_grade-equations:

        :math:`\\eta =
        \\frac{\\dot{Q}_\\mathrm{chilled,nominal}}{\\dot{Q}_\\mathrm{hot,nominal}} /
        \\frac{T_\\mathrm{high, nominal}}{T_\\mathrm{high, nominal}
        - T_\\mathrm{low, nominal}}`

    Parameters
    ----------
    nominal_conditions : dict
        Dictionary describing one operating point (e.g., operation under STC)
        of the chiller by its
        cooling capacity, its electricity consumption and its COP
        ('nominal_Q_chill', 'nominal_el_consumption' and 'nominal_cop')

    Returns
    -------
    q_grade : numerical value
        Quality grade

    """
    t_h = nominal_conditions['t_high_nominal'] + 273.15
    t_l = nominal_conditions['t_low_nominal'] + 273.15
    nominal_cop = nominal_conditions['nominal_Q_chill'] / nominal_conditions[
        'nominal_el_consumption']
    q_grade = nominal_cop / (t_l / (t_h - t_l))
    return q_grade
