Energy storage and retrieval systems and methods

A method of storing energy, comprising:

• storing a first energy storage medium at a first temperature in a first low-temperature energy storage tank, the first low-temperature energy storage tank having a pressurized gas therein;
• passing the first energy storage medium through a gradient heat exchanger;
• compressing a working fluid;
• passing the compressed working fluid through the gradient heat exchanger;
• rejecting heat from the compressed working fluid to the first energy storage medium over a temperature range between the first temperature and a second temperature higher than the first temperature in the gradient heat exchanger, thereby heating the first energy storage medium and cooling the compressed working fluid;
• storing the heated energy storage medium in a high-temperature energy storage tank at the second temperature, the high-temperature energy storage tank having the pressurized gas therein;
• balancing a pressure in the high-temperature energy storage tank with a pressure in the first low-temperature energy storage tank using a conduit in fluid communication with the pressurized gas in each of the low-temperature and high-temperature energy storage tanks, wherein the conduit passes through the gradient heat exchanger;
• exchanging heat between the pressurized gas and at least one of the first energy storage medium and/or the working fluid in the gradient heat exchanger;
• storing a second energy storage medium in a second low-temperature energy storage tank;
• circulating the second energy storage medium from the second low-temperature energy storage tank, through a substantially isothermal heat exchanger, and back to the second low-temperature energy storage tank using a pump;
• and
• exchanging heat between the working fluid and the second energy storage medium at a substantially constant temperature in the substantially isothermal heat exchanger, thereby changing a phase of the working fluid.

A method of retrieving stored energy, comprising:

• storing a first energy storage medium at a first temperature in a high-temperature energy storage tank, the high-temperature energy storage tank having a pressurized gas therein;
• passing the first energy storage medium through a gradient heat exchanger;
• passing a working fluid in a trilateral cycle through the gradient heat exchanger;
• rejecting heat from the first energy storage medium to the working fluid over a temperature range between the first temperature and a second temperature lower than the first temperature in the gradient heat exchanger, thereby heating the working fluid and cooling the first energy storage medium;
• storing the cooled first energy storage medium at the second temperature in a first low-temperature energy storage tank, the first low-temperature energy storage tank having the pressurized gas therein;
• balancing a pressure in the high-temperature energy storage tank with a pressure in the first low-temperature energy storage tank using a conduit in fluid communication with the pressurized gas in each of the high-temperature energy storage tank and the first low-temperature energy storage tank, wherein the conduit passes through the gradient heat exchanger;
• exchanging heat between the pressurized gas and at least one of the expanded working fluid and the first energy storage medium in the gradient heat exchanger;
• storing a second energy storage medium in a second low-temperature energy storage tank;
• expanding the heated working fluid;
• circulating the second energy storage medium from the second low-temperature energy storage tank, through a substantially isothermal heat exchanger, and back to the second low-temperature energy storage tank using a pump;
• and
• exchanging heat between the expanded working fluid and the second energy storage medium at a substantially constant temperature in the substantially isothermal heat exchanger, thereby changing a phase of the expanded working fluid.