def get_eta_transmission(district_type):
    """
    Returns efficency of lhn based on Fraunhofer Umsicht: Leitfaden Nahwaerme (p.51)

    Parameters
    ----------
    district_type : string
        type of district (big, medium, small)

    Returns
    -------
    eta_transmission : float
        efficiency factor for transmission
    """

    if district_type is 'big':
        eta_transmission = 0.93

    elif district_type is 'small':
        eta_transmission = 0.85

    else:   # medium
        eta_transmission = 0.9

    return eta_transmission