The model of the heat pump. On the left side is the lower temperature and on the right side the higher temperature environment. Shades of red and blue indicate temperature. Ⓐ→Ⓑ is isothermal compression, which releases heat; Ⓑ→Ⓒ is adiabatic expansion; Ⓒ→Ⓓ is isothermal expansion, which absorbs heat; and Ⓓ→Ⓐ is adiabatic compression.