One thought I had was to use double-pane vinyl slider windows for the glass sides with screens - and just remove the entire sash in summer to let the air flow when it's hot. However, this would probably not be very efficient during the heating season.

Solariums and sunspaces were very popular 20-30 years ago and I designed quite a few, in both custom passive solar residences and as additions to existing structures. The basic principles haven't changed much, but the existing architectural style typically dictate what forms are used in the massing and windows/doors selected for the fenestration. Finish materials should also complement what exists on your home.

If your home can support modern styling, consider incorporating several awning window units arranged horizontally along the base of the long vertical wall. Fixed picture window units could then be stacked directly above the awnings.

At the highest vaulted ceiling area, install a series of operable skylights - with screens. This is a classic arrangement for inducing convective air flow during summer months - perhaps combined with a couple paddle fans hung from the glazing rafters - or lateral beams.

Without question, use insulating glass everywhere and over-insulate all framed wall and ceiling cavities - paying particular attention to the installation of a moisture barrier beneath the finish material on the warm side of these surfaces.

Depending upon the type of central heating system you have, research possibilities of branching a separate zone off the furnace or boiler. Electric would be a prohibitively expensive energy source - so I would avoid that option as an add-on system.

A 20 ft deep sunroom will require careful structural engineering for the shed roof and a 2-story wall to receive it. The most successful addition designs match the pitch of the existing main roof slopes. The resulting form may appear incongruous otherwise.

This sounds like an exciting project with lots to consider - especially regarding compliance with local building codes, ordinances and covenants - or restrictions imposed by any neighborhood community boards which may exist. Done right - it will both enhance your quality of living and improve the resale value of your home.