Why can a very large body of water not store summer heat? On this page, it states "The key disadvantage of using a very large body of water to achieve heat exchange with a relatively constant temperature is that you are not able to store summer heat in that body of water – to have the benefit of retrieving those higher temperatures in winter."
Why is it so? Is it because a very large body of water would have more heat exchange with the air and hence would lose the heat gathered in summer?
But "heat exchange with a relatively constant temperature" also points in the direction of having a large body of water, so I am a bit confused.
 A: I find the statement wrong. Living in Greece  and, when younger, swimming in the sea from end of May to beginning of November, I know that the large body of water , Corinth gulf , is much warmer in November than the small body of water of a connected sea lake. So large bodies of water store heat energy of the summer much better than small bodies.
It is interesting to see the temperature of the sea water in the weather reports all winter, the sea  cools much less than air in the winter. The opposite is true in the summer. The variations is seen in these average  sea temperature per month color scaled charts.
A: I think you may be misinterpreting the statement. It is not claiming that heat of summer cannot enter a lake (and be stored in it), but rather that 'you' in your actions with the heat pump cannot 'store' useful energy into the lake.
If you had a large (say a few tens of cubic metres) insulated tank of water in your basement to use as your heat exchanger then during summer, when you don't need to heat your home, you could very efficiently pump heat from outside into the tank of water. This would raise its temperature and you could then extract the 'stored' heat (effectively for free) during winter to heat your home.
But if you use a 'large body' (or flowing body) of water for heat exchange, no amount of heat input from 'your heat exchanger' will have any significant effect on the bodys effective tempetaure. So you wont be able to extract that heat in winter.
A: The article is wrong. Consider the following: a popular form of heat pump HVAC uses coils of pipe buried in the ground and in communication with the subsurface water table. The ground water reservoir stays at an almost constant temperature year round (52 degrees F where I live) and furnishes heat in the winter and cooling in the summer via the heat pump.
Now we imagine a similar system where a simple solar array transfers heat into the cold ground water in the summer, bringing its temperature higher than 52 F, at the same time the 52 F water some distance away drives the AC function of the HVAC system of the dwelling. Then, the system runs in reverse in the winter (taking heat from the warmed-up ground water reservoir, and chilling the ground water in the nearby cold sink for the summer AC).
Eminently doable.
