  Nanoflares in quiet-Sun regions during solar cycle 24 are studied with the
best available plasma diagnostics to derive their energy distribution and
contribution to coronal heating during different levels of solar activity.
Extreme ultraviolet (EUV) filters of the Atmospheric Imaging Assembly (AIA)
onboard the Solar Dynamics Observatory (SDO) are used. We analyze 30 AIA/SDO
image series between 2011 and 2018, each covering a $400 \times 400$ arcsec
quiet-Sun field-of-view over two hours with a 12-second cadence. Differential
emission measure (DEM) analysis is used to derive the emission measure (EM) and
temperature evolution for each pixel. We detect nanoflares as EM-enhancements
using a threshold-based algorithm and derive their thermal energy from the DEM
observations. Nanoflare energy distributions follow power-laws that show slight
variations in steepness ($\alpha =$ 2.02 to 2.47) but no correlation to the
solar activity level. The combined nanoflare distribution of all data sets
covers five orders of magnitude in event energies ($10^{24} \mathrm{~to~}
10^{29} \mathrm{~erg}$) with a power-law index $\alpha=2.28 \pm0.03$. The
derived mean energy flux of $(3.7\pm 1.6)\times
10^4\mathrm{~erg~cm^{-2}~s^{-1}}$ is one order of magnitude smaller than the
coronal heating requirement. We find no correlation between the derived energy
flux and solar activity. Analysis of the spatial distribution reveals clusters
of high energy flux (up to $3\times 10^5 \mathrm{~erg~cm^{-2}~s^{-1}}$)
surrounded by extended regions with lower activity. Comparisons with
magnetograms from the Helioseismic and Magnetic Imager (HMI) demonstrate that
high-activity clusters are located preferentially in the magnetic network and
above regions of enhanced magnetic flux density.
