  We discuss the detection of large scale HI intensity fluctuations using a
single dish approach with the ultimate objective of measuring the Baryonic
Acoustic Oscillations and constraining the properties of dark energy. We
present 3D power spectra, 2D angular power spectra for individual redshift
slices, and also individual line-of-sight spectra, computed using the S^3
simulated HI catalogue which is based on the Millennium Simulation. We consider
optimal instrument design and survey strategies for a single dish observation
at low and high redshift for a fixed sensitivity. For a survey corresponding to
an instrument with T_sys=50 K, 50 feed horns and 1 year of observations, we
find that at low redshift (z \approx 0.3), a resolution of 40 arc min and a
survey of 5000 deg^2 is close to optimal, whereas at higher redshift (z \approx
0.9) a resolution of 10 arcmin and 500 deg^2 would be necessary. Continuum
foreground emission from the Galaxy and extragalactic radio sources are
potentially a problem. We suggest that it could be that the dominant
extragalactic foreground comes from the clustering of very weak sources. We
assess its amplitude and discuss ways by which it might be mitigated. We then
introduce our concept for a single dish telescope designed to detect BAO at low
redshifts. It involves an under-illumintated static 40 m dish and a 60 element
receiver array held 90 m above the under-illuminated dish. Correlation
receivers will be used with each main science beam referenced against an
antenna pointing at one of the Celestial Poles for stability and control of
systematics. We make sensitivity estimates for our proposed system and
projections for the uncertainties on the power spectrum after 1 year of
observations. We find that it is possible to measure the acoustic scale at
z\approx 0.3 with an accuracy 2.4% and that w can be measured to an accuracy of
16%.
