With modern building design a greenhouse will do a lot more than provide a place to grow green plants in winter.  A modern greenhouse can be used to significantly lower your heating bills.

A greenhouse uses the sun and transparent material such as glass or plastic to warm the air, plants and anything else inside.  A greenhouse works because the surfaces inside absorb visible and ultraviolet radiation. 

Absorbing this radiation warms the surfaces.  The surfaces emit infrared radiation.  Infrared radiation is reflected or absorbed by the glass or plastic greenhouse material.  This results in a temperature inside the greenhouse compared with the outside temperature.

People have been using this effect for a long time to warm buildings and to grow plants in winter.   

By integrating a greenhouse as a part of your house it can provide heat for warming the air inside your house and acts to insulate a wall from the cold outside.

Any pond or body of water has the potential to act as a source of geothermal energy. Even storage containers of water can store energy.

Water acts as a large thermal mass that stores heat. It is very easy to extract or store heat in water. 

An example use that is very simple to implement is to use geothermal energy from a pond to keep a driveway ice free in winter and cooler in summer. The pond can also serve as a water garden and store rainwater for watering your lawn and garden.

You need a pond that is large enough so it will not freeze solid in winter.  The rest is pretty low tech and simple.  Use efficient 12 volt pumps to pump pond water in a series of pipes underneath a drive or walkway. 

In summer, the water will be cool compared to the pavement and absorb heat, making the pavement more comfortable.  In winter the water will be warmer than freezing so it can melt ice on the drive.  If climate is very cold you will need a big pond or it will eventually freeze solid.

A pond also has the additional potential to cool water by evaporation from the surface of the pond.  This effect can be enhanced by fountains or spraying a mist on areas that need to be cooled. 

A geothermal pond can also serve as a component in a geothermal heat pump system .  The geothermal pond is used by the geothermal heat pump to dump heat in the summer and extract heat in winter.

This type of heat pump is no more complex than a standard heat pump system.  A standard heat pump system echanges heat with the outside air.  A geothermal heat pump exchanges heat with water.  Since water stores more heat than air, the geothermal heat pump is much more efficient. 

The most cost effective way to construct a geothermal heat pump system is to bury a series of pipes horizontally under the ground and/or beneath a pond.  The larger the volume of the pond the more efficient the system will be since water better than the soil at storing heat.

A geothermal pond is usually the least expensive and most cost effective way to build a geothermal heat pump system. 

An added benefit of a geothermal pond is that it can double as a recreational pond or lake for large systems or a water garden for smaller systems. In either case, excess water from the geothermal pond can be used for watering lawns and gardens with any runoff returned to the pond.

Geothermal heat pumps are almost always a cost effective alternative to a standard heat pump.  Geothermal heat pumps use 30%-70% less energy than other types of heating and air conditioning systems.

The term "geothermal" makes them sound high-tech and complex, however, this technology is relatively straightforward and simple.

The basic idea is that the temperature a few feet below the ground is very stable throughout the year. Depending on where you live this temperature can range from 45°F to  70°F.  This temperature is roughly equal to the average outside temperature for the year.

This means that just below your feet there is a relatively cool thermal mass in summer and a  relatively warm thermal mass in winter.  All that has to be done to tap this energy potential is to pass water through pipes that are in thermal contact with the earth below this temperature line.